@Senor Snarf
Win7 is a release candidate, its not a finished product, theres no way I'd even consider using it for my main operating system unless you just love trouble, and trying to resolve non resolvable issues.
Gaming wise I'm strictly a FPS [First Person Shooter] fan period, I ran into non resolvable issues with Crysis and Crysis Warhead, and FEAR2 with Steam, so after hours of frustration, for me Win7 is past history.
A friend of mine is a FSX [Flight Sim 10] fan and he has had the best FSX gaming experience so far than any other OP/SYS hes run it on yet, but FSX is a Microsoft Game, so you would expect MSFT to have covered their own product.
I'm sure Win7 will end up being great after MSFT gets all the bugs out of it, some of the AV programs available could be causing some of the issues, it seemed to run best on the Nortons but Nortons is only a 15day trial, so you end up installing others and their baggage, but you have to have some kind of AV protection.
You may even want to consider installing WinXP until Win7 is fully joy, WinXP has zero problems with any game, however your original question was between Vista and Win7, and I'd definitely go with Vista.