Unsupported resolution during boot : splashscreen problem

I'm trying to install Ubuntu 7.04 on 10 quite old Dell Optiplex GX240 (Pentium IV 1,7 GHz with Ati Rage 128 or equivalent AGP graphic card). They were under Ubuntu 6.06 without a single problem. Now, no splashscreen is visible during the boot process : the 17" LCD screen report an invalid resolution (it supports resolution up to the traditionnal 1280x1024@60 Hz). I tried to add the vga=794 option to kernel options via editing Grub's config before boot, same problem. When I try vga=791, splashscreen appears, but too big and not centered on the screen. I must precise I used alternate install CD (standard CD could not give me a X screen...)