You will get equal frame-rates in both the OS provided your PC is enough to run Windows 7 smoothly.
For example, If you have a single core processor or less than 2Gb RAM then windows XP will run the games faster as compared to Win 7. But if you have better hardware then both will give same performance.
it all depends what your main purpose in upgrading to 7 is... whether it be for office or professional use, windows 7 has the awesome compatability mode that allows you to run programs in XP! making it almost impossible for any program not to run in 7... plus with so much more added functions and advantages!
whether it be for gaming, you have the advances of direct X10 and possibly direct X11 (if your fortunate enough to own an HD5000 series card for ATI, or the GTX 400 Series for Nvidia!), visually, windows 7 is a much more practical and beautiful OS to use!
coming from experience, i upgraded from XP Pro to Vista 64 bit, and found countless problems with drivers and compatability, but now that i upgraded to windows 7 64 bit ultimate, i don't have any hassles and can do practically anything i did in XP and so so so much more!!!
windows 7 is definitely worth the investment!!!
(I should stop now, it sounds like i'm trying to sell the operating system to you!! )