Well, as you mention fps I assume you are talking frames per second in games. In most cases is Windows Xp faster in games. The selling point for Vista and games is of course DX10, but there seems to be extremely limited advantage (at this point) with DX10 and the games are very few and far between that utilizes DX10, and even those who do, can be tricked to give you the same effects in DX9.
Personally I would say, if you are going stick with Windows you might as well keep Vista on your machine as that is the future for MS. They are not going to invest any real development (if any) in XP. SP1 is shortly being released for Vista which is said to contain performance improvements.
All that said, if you have got a spare copy of XP and you prefered XP over Vista it might be worth installing it.
Bearing in mind that MS seems to force developers to release Vista only releases.