That's the minimum requirement for a complete system with a single 3870 in it
Have you considered something like this?:
Then again, there are really good PSUs available for not too much more money. Something like this corsair could easily run your rig:
Even though you can run dual PSUs... it's a pain in the rear end and I would not recommend it. I'd sell your current PSU and get that 750W Corsair. It's much better than the PSU you have in terms of quality anyways, and I'd simply recommend it EVEN if you don't plan to upgrade.