This depends greatly on what resolution you're gaming at.
I play at 1920x1080. I'm using a Sapphire 6870 and an Antec 640W PSU. And a Phenom 955 BE overclocked to 3.8, if you care.
I have all SWTOR settings maxed and only fall below playability in Warzones. But people rocking GTX 580s are also fallling below 30fps in warzones. Otherwise I'm always above 40 and quite frequently approaching 100.
Keep in mind that SWTOR still has a lot of problems. People with cutting-edge graphics solutions are still experiencing horrible performance in some cases. No matter what card you get, be prepared for some possible headaches. And stuttering. I still get a decent amount of stuttering and I think most other people do, too.