When I first upgraded to XP I was playing UT on my Radeon DDR. I was getting 70 FPS and with XP It dropped to 35, cut in half. But the game itself became smoother. It was amazingly smooth. With better drivers the FPS went back up to 70, but with no noticeable improvements in smoothness.
I don't know why, or if it will still hod true, but from that I learned that FPS surprisingly don't reflect actual playability. It may be possible that games at lower FPS may play smoother then the higher FPS in XP.
I don't know why, maybe some errors in FPS calculation,, but playability is more important then raw FPS.