If you don't want to switch out the PSU, based on my personal experience, a 5570 or 5770 crossfire runs easily well. I am switching to a 5850
I am using a 650HX corsair 80+ Bronze power supply though.
I looked up your Ultra 650Watt and found this Ultra LSP650 650-Watt Power Supply - ATX, SATA-Ready, SLI-Ready, 135mm Fan, Lifetime Warranty at TigerDirect.com
I think if this yours, it seems good, considering it has Lifetime warranty. So incase you wanted to reduce cost it might be nice.
If I knew where you lived or if your near me I sould sell a 5570 they are real nice but since i am upgrading.
FYI 5850 needs two 6 pin connectors and 5770 needs one 6pin power connector. Be careful and don't make a mistake, take it slow.