In Vista, I could max at about 200fps in Q3, but it wasn't steady. Couldn't even keep steady at 125fps cap. It also had some strange frame issues that I could never fully work out. On 7, easy 400-500, with no issues.
Haven't tried on this machine, but I was easily pulling 220fps on a single core 1.8ghz/1gb ram/Geforce 5900 on XP. I'd assume somewhere in the realm of 600+. =P
The performance is practically the same provided vista was initially setup correctly. Out of the box Windows 7 is much faster. I've been using beta since March with very few issues, just waiting for my copy to arrive.
Its been long since I was actually pleased with a microsoft product from the start :) On the performance part; it will do at least as good as Vista, probably better.
A lot of people say windows 7 just feels more stable and smooth all around so naturally it's better than Vista. Just go to the other thread polling Cinebench tests from people. Every person who posted results from Windows 7 were faster than the ones with Vista or XP.