I've run XPx86, XPx64, W7x32, and W7x64 on the same system to measure the benchmarks using all 4 OSes on the exact same hardware (the DFI-LP-X48-T3RS with 4 GB RAM). I found that both of the x64-based OSes are slightly slower in the benchmarks than the 32-bit versions. I could not actually see the differences, but further research on google showed that Windows x64 versions use Windows On Windows (WOW) to run 32-bit apps, which adds a processing layer (and time) to run. I've settled on running XPx86 and W7x64 on that machine. I use XP to do general work and play older games (DX9), while I use the W7x64 for gaming - it has DX11.