Your best bet is to go to several PC maker's websites, and price out computers. The minimum requirements for games are easily found, and so are video card rankings. Cheapest depends on who's running a sale at the moment, so check out Dell, Gateway, HP/Compaq, Acer, etc. Newegg sells full PCs also, IIRC. If you're a student you can usually get a pretty good discount through your school too.