Earlier, I posted the link to the "3 Gb switch" how-to site. It has just come to my attention that this should not be performed when using a video card with 512 mb of RAM.
The reason is that the video card requires double the amount of RAM it has (512 Mb x 2=1Gb) from Windows XP. So with 2 Gb used for Revit and 1 Gb used for Windows that leaves 0 Gb available for the video card. Not good.
64 bit computing won't come too soon in my opinion!