gainward's 5600 ultra is the best. Its 50mhz faster on core and mem i believe. I used the PNY 5600U and it's just as fast, but without VIVO.
Be sure to get the Revision 2, new core design (flip-chip), and faster clocks (400+/800+). Revision 1's are Abit, Albatron, Chaintech and a few others.