Comment 4 for bug 150248

Jean-Denis: You would lose around a gig with Vista 32 as well. I don't think it has PAE (which allows 32-bit to have 4 GB). Apparently Ubuntu i386 doesn't either. PAE is a bit unstable so it's preferable just to use 64-bit to get full memory support.

It happens because the upper portion of the 4 GB virtual address limit needs to be reserved. There needs to be space for devices to read/write info to the bus. I don't know all the specifics although it is a well-documented issue that you can only see 2.5-3.0 GB with a 32-bit system (depending on how much RAM is on your graphics card, etc).