Vista is the way to go since it's a gaming system since DirectX 10 is something that you can only get with Vista. Right now there's a lot of new games that are coming out that take advantage of DirectX 10, so definately go Vista.
The people that are yelling out XP are those that don't understand gaming needs for a true gaming system
I myself run Vista and I've never had any problems with it, it's a great system.