VRay all the time, I do like the Mental Ray Final Gather, fast and looks like GI, but I read the 'restrictions' for MR as apposed to VRay, robust and reliable like me LOL.
I seriously advise getting 64 bit XP, 32 bit XP can address max 4 gig of memory, 64 bit can address much more, at least 16 gb, most apps in 32 bit OS will fold at 1.6 gb usage, Max certainly does, so I think you have reached a memory limit.
Laters
spike