I work at a state university (nonprofit, of course) and I checked our IT store. The cheapest x64 Vista would be an upgrade disk (70 bucks+ tax) which is actually x32 but you can use the key to send away for an x64 Vista DVD. Amazon has the same thing for 75 bucks (no tax, free shipping).

My hardware at least passes the test to run x64 (EVGA nforce 680i mobo, onboard soundcard, 2 x 8800GTS's in SLI running a DELL 3007, SATA RAID, 4GB PC3200 though I have 2GB more sitting around). However, I cringe at the thought of doing MS's upgrade path from XP32 -> Vista64 and finding everything broke, BSOD's what have you. My experience with installing OS's on top of other OS's (as opposed to just wiping the HD and going with a fresh install) has never been good. The thought of having to reinstall WoW (whether off an image or fresh+patches) also makes me cringe.

I'm actually less hesitant to try to go x32 XP to x64 XP because I 'heard' that it's easier to reverse that move should things go awry. (And by reverse, I don't mean "format C:/")