Let's put this bit of FUD to rest.
Fact: Windows Vista on launch was around 5-10% slower than Windows XP for gaming, this was largely due to video drivers not being as mature.
10% isn't even noticeable, I'd happily trade 10% for video drivers which don't take out the entire system when they crash thanks to them running in User Mode in Windows Vista instead of Kernel Mode like they do in Windows XP.
Fact: That gap was closed months ago, and now in many benchmarks Windows Vista is faster.
They also benchmarked World in Conflict, where the two were dead level.
Conclusion: there's no difference, and if somebody wants to get picky and argue Windows XP must be faster, kindly point them in the direction of some benchmarks.