It seems that people are always saying the newer OS runs faster. I decided to do some quick benchmarks on my brother's rig to see if that is the case.
e3400 @ 3.7 GHz
4 gigs ram
8800 gts 320 mb
Test game: Blacklight Retribution "Training room" (CPU bottlenecked in this scenario)
Did a clean install for each OS and used 304.79 BETA drivers. Used Fraps benchmark tool and did the benchmark 3 times for each OS.
Windows XP SP3 32-bit: 193 FPS average
Windows 7 SP1 64-bit: 145 FPS average
Windows 8 RTM 32-bit: 139 FPS average
Max/min fps were all about 30-40 FPS above/below average FPS.
Conclusion: I'm not sure what the results are for GPU bottlenecks, but if you are CPU bottlenecked and your PC is older, then Windows XP is the way to go.