this site will give you an idea which games you can run on your set up: Can You Run It?
As whether gaming is as good on 7 as it is on xp? Well it depends on the game but everything I've run so far as been fine. Don't forget that with 7 you get Dx11 too. Ok it's in it's infancy (Dx11) but your 'end-user' experience will be greatly enriched using 7... (in my opinion of course )
One of the biggest gaming advantages in 7 is DX11 support. There will be no problems at all running latest 3D games in Windows 7, performance (FPS) will be much the same as in XP. However, some rare old games like Myst 5 End of Ages won't run in 7 (even in compatibility mode).
Good read (includes gaming benchmark comparisons):
I'm running Lord of the Rings Online, (they just went free to play) and having a blast.
I'm running it in DX11 with all most everything turned on, even anti aliasing and I'm seeing frames rates as high as 250 fps with a low around 150.
I have also run, Age of Conan, Dragon Age, Tomb Raider Underworld, The Witcher, and WOW on my new Windows 7 64 bit computer.
These games also ran fine on my old 32 bit computer when I converted it from XP to Windows 7 32 bit.