For what it's worth, I've tried Dead Space, Painkiller, all Infinity Engine games, the last 3 Tomb Raider games, Jagged Allliance 2, Europa Universalis III, NWN2, The Witcher, VMBL, Kotor1 & 2, Pirates!, MS Flight Simulator X, X3, BF1942 and a handful of titles that I can't remember without problems on 64 bit quad core architecture. Only issues were gfx related and could be fixed in the ATI control panel
I still have a handful of "very" old games that needs testing. Moo2 didn't work. I am not sure that X-Com 3, Carmageddon and X-Wing vs. Tie-Fighter will work, still have to try those.
In short, I think people attribute mystical destructive powers to 64 bit