Think this all boils down to drivers.
Wait till some solid stable drivers for sound / graphics come out, and I bet 99% of games that work in XP will work in Vista no problem.
As the threads going, for an example, 1 person has to run Battlefield 2 with administrator rights, I dont, it just works.
GTA SA works like a dream on my Ultimate x86, but others cant run it.
Give it a month or 2 for drivers, and they'll work.
P.S.
I know it's a pain waiting for drivers thats certified, but there you go, cant be helped.
Does'nt help that Microsoft foobered things in Vista either, like removing the Hardware Acceleration Layer thing on sound, dont ask me for techie terms right now, I have had a jar or 3

why Creative have no drivers yet, except for beta's.
But everyone should get the gist of what i'm saying.