OS is usually a matter of opinion might not be much help but..when you order a new comp gives you an option of getting vista and upgrading to windows 7 for a low price or free. XP is good for gaming but only good with 2gb of ram if you put 4 it will read but you wont see a difference. If you wanna see speed with your ram and also have better video since VISTA has DirectX10 id go with vista. I still use XP im happy with it but i like how vista looks, you could also wait for windows 7 to come out which beats all IMO. hope that helped