Here are a few benchmarks between my x32 and x64 XP Pro installs:
System:
Gigabyte K8NS Ultra 939 (nforce3 Ultra)
SB Audigy 2 ZS
AMD Athlon64 4000+ (San Diego)
WD Raptor 36gb (1st gen)
Drivers latest for all devices, save the motherboard chipset (performance differences outlined below), I made sure that previous driver versions were fully removed before trying another set (had a backup of the windows install). V-sync disabled on all tests and I used all of the drivers (save audio) bundled with each driver install package during setup.
Far Cry (Max Details @ 1600x1200, latest patch, Direct3D, no FSSAA or AF, SM at default using FarCry benchmark utility from Hardware OC):
FarCry Steam map demo (by hardware OC):
x32 (5.10 nvidia WHQL): 59.09 FPS
x64 (stock windows drivers): 53.72 FPS
x64 (nforce4 AMD drivers, stock windows GART driver, latest on nvidia's site): 54.56 FPS
x64 (Nforce Unified Remix 11.16): 54.34 FPS
FarCry Volcano map demo (by Ubisoft):
x32 (5.10 nvidia WHQL): 86.77 FPS
x64 (stock windows drivers): 79.69 FPS
x64 (nforce4 AMD drivers, stock windows GART driver, latest on nvidia's site): 79.59 FPS
x64 (Nforce Unified Remix 11.16): 79.91 FPS
3Dmark 2006 (fee edition):
x32 (nvidia WHQL): 2701
x64 (stock windows drivers): 2632
x64 (nforce4 AMD drivers, stock windows GART driver, latest on nvidia's site): 2649
x64 (Nforce Unified Remix 11.16): 2646
CounterStrike: Source stress test (note that the 64-bit version of CS: Source was used on the 64-bit windows install) (max details on everything, 6x FSAA, 16x AF):
x32 (nVidia WHQL): 88.53 FPS
x64 (stock windows drivers): 87.25 FPS
x64 (nforce4 AMD drivers, stock windows GART driver, latest on nvidia's site): 87.69 FPS
x64 (Nforce Unified Remix 11.16): 87.34