Neither. 5830 is already clocked pretty high, and is crippled due to it's low ROP count. It performs closer to the 5770 than the 5850 in most cases and it uses more power than the 5850. As for the 5770 it's basically half a 5870. If you want something to overclock to get to the level of a 5870 then you need to either go with a 5850 or a GTX460 1GB with an aftermarket cooler. You could also just wait for the 6700 cards to come out and drive down the prices a little.
So does that mean 2 5770s equal a 5870