what do you need, do you need a monitor as well? a kb + mouse? cd roms? cases?
if you need a FULL system (mointor, KB mouse, dvd drive, case, etc.), with everything and lowest end 4 processor (quad core) then the lowest gfx, you'll be looking at 600-700 USD for a desktop i'd wager.
BTW usually we start with a budget and what you want to do with it (gaming, office work, home theater, home theater with gaming, internet browsing, programming, etc.) and what you currently have and can be used again and then we try to tell ya if it's possible at that price and what is best at the price, and not the other way around.