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.
From what you have asked, you can purchase a computer with Win7 and a quad processor fairly cheap. With that, it depends on your specific computer needs as to the other components needed in the computer which will determine the overall price. If you visit bestbuy.com, dell.com, or hp.com you will see computers priced around $600-$700 with decent configurations. Again, it really depends on your specific needs to determine an average price.
Basic configurations of hp and dell models can begin in the low 500's, however processor choices will be limited to previous generations in that range (eg. Phenom v Phenom II). I think video capability will be your implication. You can do alright with 3 gb of ram.