I have 4GB of ram on my new build. I had installed XP Pro 64 on a previous system with 4GB but I had some problems with programs working/installing on the XP Pro 64. I built this new comp for Crysis and other high end games. I would like X10 support so i can play Very High on Crysis. I know there are limitations on 32 bit OS but would I see a huge decrease in performance if I installed normal XP with 4GB (i think it can only use 3.5GB) vs Vista or the 64 bit versions?
Well.. If you really want DX10, then it's Vista or nothing. XP doesn't support DX10 - only up to 9C.
If you want 4GB of RAM which is entirely visible/usable, then you need a 64 bit OS.
The only OS which meets all of the above requirements is Vista 64. I have it, and contrary to popular belief/FUD, it's been pretty much bullet proof for the 6 months I've been running it. And I haven't had any troubles finding drivers, either. The only very common thing I'm aware of which wasn't supported for a long time was X-Fi sound cards. Why?? Because a bunch of monkeys high on angel dust and valium can write better drivers than the developers at Creative can. (My Humble Opinion, of course)
If you don't "need" the above, then run XP. Less overhead will get you a few more frames per second.
Of the choices you list, I'd use XP64 Pro. It does everything 32 bit XP does and offers 64 bit computing. The old driver issues are gone now, thanks to Vista 64, and XP64 is very stable. Sure no DX10, but it will be a long time before lots of DX10 games will be out, much less required. You can also use the whole 4 gig of ram, even 8 gig if you want, though only some business apps use that much.
If you decide to get Vista, upgrade to the 64 bit edition. It doesn't cost much and it unlocks a lot of potential, besides letting you use the whole 4 gig of ram.