Just had to chime in here...
What game(s) are you having compatibility issues with ?
I am a dedicated gamer and use Vista 64-bit and have had no issues running all the big titles .. Bioshock 2, Mass Effect 2, Dragon Age Origins, Gothic 4, Supreme Commander, Far Cry 2 ......
A lot of the newer titles make use of 4GB+ of RAM, multiple CPU cores and most importantly 64-bit
Vista... now that's a horse of a different color. I don't think the XP X64 would be so nice to you.
Did Halo 3 ever get made for the PC? I kind'a liked that one (v2 that is).