The upcoming 28nm graphics cards will need less power. The 7970 can run on a 500w psu, and I expect the GTX680 to be the same.
But, I like to overprovision a bit so that the psu does not need to operate at max and get noisy under load.
I think a 650w unit would be appropriate.
For quality, look at Seasonic, Corsair, PC P&C, Antec or XFX.
For value, you just can't beat this Antec EarthWatts EA-650 GREEN 650W for $60 after rebate. I know it is not modular, but you are going to be using most of the leads anyway.http://www.newegg.com/Product/Product.aspx?Item=N82E16817371044
For modular, here is a Seasonic 620w unit for $90:http://www.newegg.com/Product/Product.aspx?Item=N82E16817151095