XoticPC still allows you to customize with XP, but I don't really recommend it. In the grand scheme of things, the performance difference between the 3 OS' with current drivers is pretty minimal and most likely won't even be noticed. And unless you keep tabs on your frame rate, you aren't going to see a difference.
I've used all 3 Operating Systems and have liked all three. XP was good, but compared to Vista and 7 it just seems very basic. In case you haven't run Win7, it is rather smooth and has evened out some of the issues people were complaining about with Vista.