I actually might stay with XP. Easier to use and less clutter + faster.
The gaming performance between Windows 7 and XP on a decent machine is negligible:
http://www.firingsquad.com/hardware/windows_7_gaming/page5.aspHowever, Drivers have improved since then, so the gap should have closed somewhat.
However, from testing Windows 7 with 768mb of memory, an Athlon 2400+ and a Radeon 9550, it was an extremely enjoyable experience, however XP was significantly faster in a game because whatever way you put it, Windows 7 does have a higher memory foot print over Windows XP, and the game (This case, Need for Speed: Most Wanted) wanted more memory than what could be given away, the end result was an FPS crawl.
However, that was an extremely old system, if it had the Ram I doubt I would have seen a performance difference between Windows XP and Windows 7.
That also brings up another point of System Maintenance, most people "Install and Forget" there software, over-time... Windows get bloated and slower, this is a situation that down-grading to Windows XP cannot fix, as you will be in that same situation again in the end, best way to mitigate that is to change how you use your system.
Plus, Windows 7 just makes life easier, Love the search, mouse gestures, how a window snaps depending on what side of the screen you drag it to, the faster boot and shut down and general responsiveness due to the caching system...