I don't really get why people say XP uses less resources then Windows 7. Running taskmanager and process explorer on my Win7 PC shows it's using approximately 1% of CPU usage (which is by taskmanger itself), practically non-existant disk IO (occasionally a couple of kilobytes) and has gigabytes of free ram. XP does do less then Windows 7, but Windows 7 does not sit there eating resources when idle (and if it is it's because you have extra apps/services running in the background).
XP ! Windows Xp is the best... but u must have good configuration to play heavy games.. and windows 7 is not considered much as it already is kinda heavy so eats up a lot of ram and etc, not allowing games to access it..
Windows XP requires less RAM so the performance of your games will be better, but if you have a decent system then you should go for Vista.
He want's to run new games, so Win 7 is the best of course. XP is outdated. You won't have DirectX11 on XP.