If you have any say in the power supply, the Corsair CX500 is a very good power supply.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027
The 470 is the fastest of those cards, but I would get the 6850 if I were you. It performs about the same as the 7770, but it comes with Dirt 3 instead of Dirt Showdown, which kinda really sucks. http://www.newegg.com/Product/Product.aspx?Item=N82E16814125353
If you could find a 470 for around this price, it might actually be a good deal though, but the GTX 400 series of cards ran pretty hot, and I'm not sure of the ventilation deal of the Dell cases.