550W - 600W for current build. If you plan to SLI/xFire in the future I would opt for 750 W - 850W.
I would switch your GPU though to Radeon 7850. Performance is similar but cheaper, less heat and more power efficient.
Ugh, i have a 5870 HD at the minute and i've always seemed to get errors in games etc. that are caused by having an ATI card. I'm definately switching back to Nvidia.