What ACE said.., games like Just Cause 2 for example already dropped XP support, I assume this is going to happen for a lot of games in the next couple of years. XP also doesn't support directx 10/11. But it depends on what you prefer, if you rather stick with Windows XP cause you're used to it, then that's okay, both OS's are perfectly fine for gaming, unless you really want those directx11 graphics (personally I don't see much of a difference, not many games use directx11 yet anyways).
Edit: you might have problems activating win7, unless you got a legal key, I'd go with XP for now if I were you until you manage to get win7 legal.