Here's a trial build, $561 with $3 shipping. It uses your choice of CPUs, but with a 770 motherboard combo, the GTS450 with the operating system. A nice case for $15 less then your case choice and a 500w Antec power supply- a bronze rated supply.
GPU: Powercolor AX5770-- http://www.newegg.com/Product/Product.aspx?Item=N82E168...
This is $10 cheaper than the GTS 450 dirtmountain posted, and is quite a bit faster. Also, it uses less power and takes up less space, as it's a single slot card. I'm also recommending this because you'll be able to put a second one in the motherboard I suggested and give yourself a nice Crossfire setup.
I use this card, and I can say without a doubt it's amazing. I can run Crysis Warhead on Enthusiast (without AA) at 1600x1200 and get 30+ frames constantly. This is with a Pentium D 945 and 2GB of RAM, mind you. I'm trying to finish getting all the parts for a new i5 760 build, but it's all getting me by just fine for now.
About the GTS 450 though, both of those are factory overclocked models that cost well over $150-$160. The 5770 I chose is the cheapest one available (~$120, new), and still costs less than those two cards.