I can attest that 5830's seem to BEG you to overclock them. A little RBE work and I've pushed mine as high as 320 Mhash/s stable.
I agree that the 5830 are awesome. I was going to get a couple 5850 but couldn't find any, then newegg had their sale on 5830...
Hard to beat 300+MH for $100-110.