I have to agree with the other guys here, anything under $500 its near impossible to beat manufactured ones, especially if you are willing to wait for a great deal. I've seen a refurbished quad core, 8GB DDR3, 1TB, Blu-Ray, halfway decent video, Windows 7 for $400. Very difficult to get near that even if you wait for deals on the components (especially if you are paying OEM for windows 7).
That said, I built my most recent computer (about $600 in parts) and it was better than anything deals I could find on a manufactured one.
That said, I built my most recent computer (about $600 in parts) and it was better than anything deals I could find on a manufactured one.