5770 xfire. Still recommends 600+ with 55 amps on the 12v rail. 36 amps on the ocz psu if you have the modxstream one.
Pulls 400 at least at load which doesn't leave you much room to overclock your cpu if you have any intentions in the future. I was also looking at an xfire setup with my replaced 6770 and had the same psu. Did a little research myself when I was contemplating it.
I don't want to discourage you but I don't want you to blow your system. If you can go with a 7800 series/ 6950/ GTX 560 ti (or non ti)/ 6870. I've seen the 6870 going for the low 100's on ebay.