Suggestions for a Good Gaming Desktop?

Alright so my situation is that my laptop is supposed to be impossible to change out graphics cards which I find very hard to believe. I'm looking for a good gaming desktop for maybe 600 to 700 bucks since my current laptop is simply terrible for gaming. I'd prefer a great desktop refurbished over an overall okay new gaming desktop. I've been told ASUS, Gateway, and Alienware are all good brands although Alienware is rather expensive. I want a desktop that has the capability of running GTA IV smoothly on normal settings. I also prefer an NVidia graphics card, but if you know one better for the price range that would be appreciated.