xorg-server can not support multiple monitor

I want to expand my 19' display to another 17' display ,but Ubuntu show me the message :
"
Could not apply the stored configuration for monitors
Request size (2390,768) exceeds 3D hardware limit(2048,2048), you muset either rearrage the display so that they fit within a (2048,2048) square or select the Ubuntu 2D session at login."

but my
 xrandr --verbose

show the following:

Screen 0: minimum 320 x 200, current 1024 x 768, maximum 4096 x 4096
VGA-0 connected 1024x768+0+0 (0x56) normal (normal left inverted right x axis y axis) 320mm x 250mm
 Identifier: 0x51
 Timestamp: 9817009
 Subpixel: no subpixels
 Gamma: 1.0:1.0:1.0
 Brightness: 1.0
 Clones: DVI-0
 CRTC: 0
 CRTCs: 0 1
 Transform: 1.000000 0.000000 0.000000
             0.000000 1.000000 0.000000
             0.000000 0.000000 1.000000
            filter:
 EDID:
  00ffffffffffff000ce3721700000000
  17110101682019a02800bda057499b26
  10444afffe0045593159615981800101
  010101010101000000fd0032821e480b
  000a202020202020000000fc00313737
  3245480a202020202020000000ff004d
  373232344335353230323632000000fe
  0058462d37740a202020202020200085
 load detection: 1 (0x00000001) range: (0,1)
  1280x1024 (0x54) 108.0MHz +HSync +VSync
        h: width 1280 start 1328 end 1440 total 1688 skew 0 clock 64.0KHz
        v: height 1024 start 1025 end 1028 total 1066 clock 60.0Hz
  1024x768 (0x55) 94.5MHz +HSync +VSync
        h: width 1024 start 1072 end 1168 total 1376 skew 0 clock 68.7KHz
        v: height 768 start 769 end 772 total 808 clock 85.0Hz
  1024x768 (0x56) 78.8MHz +HSync +VSync *current
        h: width 1024 start 1040 end 1136 total 1312 skew 0 clock 60.1KHz
        v: height 768 start 769 end 772 total 800 clock 75.1Hz
  1024x768 (0x57) 75.0MHz -HSync -VSync
        h: width 1024 start 1048 end 1184 total 1328 skew 0 clock 56.5KHz
        v: height 768 start 771 end 777 total 806 clock 70.1Hz
  1024x768 (0x58) 65.0MHz -HSync -VSync
        h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.4KHz
        v: height 768 start 771 end 777 total 806 clock 60.0Hz
  1024x768 (0x59) 44.9MHz +HSync +VSync Interlace
        h: width 1024 start 1032 end 1208 total 1264 skew 0 clock 35.5KHz
        v: height 768 start 768 end 776 total 817 clock 43.5Hz
  832x624 (0x5a) 57.3MHz -HSync -VSync
        h: width 832 start 864 end 928 total 1152 skew 0 clock 49.7KHz
        v: height 624 start 625 end 628 total 667 clock 74.6Hz
  800x600 (0x5b) 56.2MHz +HSync +VSync
        h: width 800 start 832 end 896 total 1048 skew 0 clock 53.7KHz
        v: height 600 start 601 end 604 total 631 clock 85.1Hz
  800x600 (0x5c) 50.0MHz +HSync +VSync
        h: width 800 start 856 end 976 total 1040 skew 0 clock 48.1KHz
        v: height 600 start 637 end 643 total 666 clock 72.2Hz
  800x600 (0x5d) 49.5MHz +HSync +VSync
        h: width 800 start 816 end 896 total 1056 skew 0 clock 46.9KHz
        v: height 600 start 601 end 604 total 625 clock 75.0Hz
  800x600 (0x5e) 40.0MHz +HSync +VSync
        h: width 800 start 840 end 968 total 1056 skew 0 clock 37.9KHz
        v: height 600 start 601 end 605 total 628 clock 60.3Hz
  800x600 (0x5f) 36.0MHz +HSync +VSync
        h: width 800 start 824 end 896 total 1024 skew 0 clock 35.2KHz
        v: height 600 start 601 end 603 total 625 clock 56.2Hz
  640x480 (0x60) 36.0MHz -HSync -VSync
        h: width 640 start 696 end 752 total 832 skew 0 clock 43.3KHz
        v: height 480 start 481 end 484 total 509 clock 85.0Hz
  640x480 (0x61) 31.5MHz -HSync -VSync
        h: width 640 start 664 end 704 total 832 skew 0 clock 37.9KHz
        v: height 480 start 489 end 491 total 520 clock 72.8Hz
  640x480 (0x62) 31.5MHz -HSync -VSync
        h: width 640 start 656 end 720 total 840 skew 0 clock 37.5KHz
        v: height 480 start 481 end 484 total 500 clock 75.0Hz
  640x480 (0x63) 30.2MHz -HSync -VSync
        h: width 640 start 704 end 768 total 864 skew 0 clock 35.0KHz
        v: height 480 start 483 end 486 total 525 clock 66.7Hz
  640x480 (0x64) 25.2MHz -HSync -VSync
        h: width 640 start 656 end 752 total 800 skew 0 clock 31.5KHz
        v: height 480 start 490 end 492 total 525 clock 60.0Hz
  720x400 (0x65) 35.5MHz -HSync -VSync
        h: width 720 start 738 end 846 total 900 skew 0 clock 39.4KHz
        v: height 400 start 421 end 423 total 449 clock 87.8Hz
  720x400 (0x66) 28.3MHz -HSync +VSync
        h: width 720 start 738 end 846 total 900 skew 0 clock 31.5KHz
        v: height 400 start 412 end 414 total 449 clock 70.1Hz
DVI-0 connected 1024x768+0+0 (0x56) normal (normal left inverted right x axis y axis) 410mm x 230mm
 Identifier: 0x52
 Timestamp: 9817009
 Subpixel: horizontal rgb
 Gamma: 1.0:1.0:1.0
 Brightness: 1.0
 Clones: VGA-0
 CRTC: 0
 CRTCs: 0 1
 Transform: 1.000000 0.000000 0.000000
             0.000000 1.000000 0.000000
             0.000000 0.000000 1.000000
            filter:
 EDID:
  00ffffffffffff00410c3dc001010101
  1715010380291778ee69b5a655489c25
  145054bd4b0081800101010101010101
  010101010101662156aa51001e30468f
  33009ae61000001e000000fd00384c1e
  5311000a202020202020000000fc0050
  68696c69707320313931454c000000ff
  004637413131323330303632323100f8
 load detection: 1 (0x00000001) range: (0,1)
  1366x768 (0x67) 85.5MHz +HSync +VSync +preferred
        h: width 1366 start 1436 end 1579 total 1792 skew 0 clock 47.7KHz
        v: height 768 start 771 end 774 total 798 clock 59.8Hz
  1280x1024 (0x68) 135.0MHz +HSync +VSync
        h: width 1280 start 1296 end 1440 total 1688 skew 0 clock 80.0KHz
        v: height 1024 start 1025 end 1028 total 1066 clock 75.0Hz
  1280x1024 (0x54) 108.0MHz +HSync +VSync
        h: width 1280 start 1328 end 1440 total 1688 skew 0 clock 64.0KHz
        v: height 1024 start 1025 end 1028 total 1066 clock 60.0Hz
  1024x768 (0x56) 78.8MHz +HSync +VSync *current
        h: width 1024 start 1040 end 1136 total 1312 skew 0 clock 60.1KHz
        v: height 768 start 769 end 772 total 800 clock 75.1Hz
  1024x768 (0x58) 65.0MHz -HSync -VSync
        h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.4KHz
        v: height 768 start 771 end 777 total 806 clock 60.0Hz
  800x600 (0x5d) 49.5MHz +HSync +VSync
        h: width 800 start 816 end 896 total 1056 skew 0 clock 46.9KHz
        v: height 600 start 601 end 604 total 625 clock 75.0Hz
  800x600 (0x5e) 40.0MHz +HSync +VSync
        h: width 800 start 840 end 968 total 1056 skew 0 clock 37.9KHz
        v: height 600 start 601 end 605 total 628 clock 60.3Hz
  640x480 (0x61) 31.5MHz -HSync -VSync
        h: width 640 start 664 end 704 total 832 skew 0 clock 37.9KHz
        v: height 480 start 489 end 491 total 520 clock 72.8Hz
  640x480 (0x62) 31.5MHz -HSync -VSync
        h: width 640 start 656 end 720 total 840 skew 0 clock 37.5KHz
        v: height 480 start 481 end 484 total 500 clock 75.0Hz
  640x480 (0x63) 30.2MHz -HSync -VSync
        h: width 640 start 704 end 768 total 864 skew 0 clock 35.0KHz
        v: height 480 start 483 end 486 total 525 clock 66.7Hz
  640x480 (0x64) 25.2MHz -HSync -VSync
        h: width 640 start 656 end 752 total 800 skew 0 clock 31.5KHz
        v: height 480 start 490 end 492 total 525 clock 60.0Hz
  720x400 (0x66) 28.3MHz -HSync +VSync
        h: width 720 start 738 end 846 total 900 skew 0 clock 31.5KHz
        v: height 400 start 412 end 414 total 449 clock 70.1Hz
S-video disconnected (normal left inverted right x axis y axis)
 Identifier: 0x53
 Timestamp: 9817009
 Subpixel: no subpixels
 Clones:
 CRTCs: 0 1
 Transform: 1.000000 0.000000 0.000000
             0.000000 1.000000 0.000000
             0.000000 0.000000 1.000000
            filter:
 tv standard: pal
  supported: ntsc pal pal-m pal-60
             ntsc-j scart-pal pal-cn secam
 load detection: 1 (0x00000001) range: (0,1)
^_^crazier@Lab:~$

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: xorg 1:7.6+12ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-26.41-generic-pae 3.2.19
Uname: Linux 3.2.0-26-generic-pae i686
.tmp.unity.support.test.0:

ApportVersion: 2.0.1-0ubuntu10
Architecture: i386
CompizPlugins: [core,composite,opengl,decor,move,mousepoll,resize,grid,vpswitch,imgpng,regex,compiztoolbox,place,commands,session,gnomecompat,unitymtgrabhandles,snap,animation,wall,workarounds,fade,expo,scale,ezoom,scaleaddon,unityshell]
CompositorRunning: compiz
Date: Thu Jul 5 15:38:56 2012
DistUpgraded: Fresh install
DistroCodename: precise
DistroVariant: ubuntu
DkmsStatus: vboxhost, 4.1.18, 3.2.0-26-generic-pae, i686: installed
ExtraDebuggingInterest: Yes, whatever it takes to get this fixed in Ubuntu
GraphicsCard:
 Advanced Micro Devices [AMD] nee ATI RV370 5B60 [Radeon X300 (PCIE)] [1002:5b60] (prog-if 00 [VGA controller])
   Subsystem: Advanced Micro Devices [AMD] nee ATI Device [1002:1002]
   Subsystem: Advanced Micro Devices [AMD] nee ATI Device [1002:1003]
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
MachineType: System manufacturer System Product Name
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.2.0-26-generic-pae root=UUID=2af08508-e607-4d17-913b-66ffd60b3034 ro quiet splash libata.force=1:pio4 vt.handoff=7
SourcePackage: xorg
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 05/20/2008
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 0309
dmi.board.asset.tag: To Be Filled By O.E.M.
dmi.board.name: P5KPL-AM
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: x.xx
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr0309:bd05/20/2008:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP5KPL-AM:rvrx.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer
version.compiz: compiz 1:0.9.7.8-0ubuntu1.1
version.libdrm2: libdrm2 2.4.32-1ubuntu1
version.libgl1-mesa-dri: libgl1-mesa-dri 8.0.2-0ubuntu3.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 8.0.2-0ubuntu3.1
version.xserver-xorg-core: xserver-xorg-core 2:1.11.4-0ubuntu10.3
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.7.0-0ubuntu1.2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:6.14.99~git20111219.aacbd629-0ubuntu2
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.17.0-1ubuntu4
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:0.0.16+git20111201+b5534a1-1build2