Get Windows 7, you won't regret it, its great. Plus you should get a small increase in gaming performance compared to XP.
I downgraded from Win7 back to WinXP due missing possibility to have mouse window focus (
without bringing to front/z-order change) which NView used to provide. I tried some external software to bring back that functionality, but it messed up everything.
Imagine if Win8 forced you to use a table input device over a mouse (as seen in the Win8 commercials/infos). That's about the same impact for those used to proper input systems.