I've been running XP Pro 64 for the last ~10 months or so exclusively. I started out dual-booting w/ 32bit in case there were any problems, but for the last 10 months or so I've had no reason to use 32bit. Even iTunes is 64bit now.
I've used nVidia and ATI cards, and play everything from the Fallout Series to FEAR. Oblivion, HL2, CS, Quake, UT2K3-2K4...you know, it'd probably be faster to say that I haven't had a game yet with any problems in 64bit.
There were even a few articles back when XP Pro 64 came out indicating that some 32bit games performed faster under XP 64. With all of the 64bit patches that've been released since then (Far Cry, for example) - I seriously doubt that you'd be able to find a clear case where a game performs better under 32bit.