Corsair 650Watt..you could do an HD5850 crossfire, but then again, that's the same price as an HD5970. An HD5830 crossfire would outperform your HD4870x2, especially if overclocked, but then you'd need a new PSU.
Ultimately, an HD5970 is the only thing worth it. ATI cards do physics just fine, and DX11 is only worth it if the games you play use it.