Simply deactivate the UAC control at your user account and vista never asks you again..If you have the performance vista is a charm for games if you don't have it use xp that's my opinion.
If you want to play new games then vista for sure. But if you got a bunch of old games that you would like to play, then xp
Vista for sure on games. Also, if you can wait Windows 7 will be supporting Direct X 11 soon. If you need a new video card, I suggest going with something from ATI cause of the best bang for buck performance and more pixel shaders.