For what its worth I run my 4 shaman off a AMD dual core,4gig of ram (sorta), Vista 32bit and a not so great 512mb nvidia card (geforce 8600 maybe?) - performance isn't perfect in shatt but AV brawls have been just fine. I did notice the response time being an issue in arenas when trying to 5box.

erm... that doesn't really answer your question at all I guess

I still really prefer XP over vista, but don't have experience with the 64 bit of either.