I say XP because the 9600GT isn't a card that will allow you to max out stuff on DX10, however it will let you max out some stuff on DX9. Also, with 2GB of RAM you have much more room in XP than in Vista.
Ya thats what i was thinking, cuz i have the money for 2 more GB i just never got it cuz i no xp only uses 2.. so i think ill get 4 gb and use vista 64. and anyway if i ever have problems with vista ill just roll back to xp