Bitmob had a chat with Jason Cross, the former editor of ExtremeTech, about how good Windows 7 is for gamers. You can read it over here.
Bitmob: At this point, can you recommend Windows 7 as a gaming platform?
JC: I'd almost insist on it. Windows XP is old enough that running it is sort of a security risk, and you can't run DX10 or DX11. Windows 7 is as fast or faster than Vista in most games, has a lot of nice interface improvements, runs better on older hardware, and has eats up less of your available memory, so your game has more [memory] to use. It's hard to find a downside, other than obviously the cost of the OS itself.
If you're running a system that's four-plus years old-Pentium 4 or Athlon 64 CPUs, less than 2GB of RAM, GeForce 7 series or Radeon 1000 series [videocard] or earlier -- I wouldn't go upgrading the OS and expect miracles. Save your money for the new computer you'll need to meet the minimum system requirements of games anyway and get Win7 with it.
If your system is fairly modern -- dual- or quad-core CPU, 2GB of RAM or more, GeForce 8 series or Radeon 2000 series [videocard] or better -- I'd recommend Windows 7.