Actually, vnaut raises a very good point. What sort of budget are you looking at spending? In my opinion, if you're looking to spend $650 or less, you're better off buying a system off the shelf, plus you get the tech support included. It's only after about the $650 mark do you start to see a savings in building your own.
Of course, if it's more about the learning experience than the cost, building is always better.
-Wolf sends