I agree I run multiple 5850's really good cards and can be seriously OC'ed for some very good Mhashs, I avg around 330 - 350 and you can get more but I want to OC my cards too much.
I OCed my 5850 to 970/325 and am getting about 400mh/s.
I third the 5850s. They strike a good balance between the low hardware cost of a 5830 and the mining power of a 5870. Add into the mix that they are lively overclockers and relatively low power and you have a real winner. They'll run well at very low voltages and happily give you 300 MH/s while being pretty cool and quiet and they can perform when needed (I've managed as high as 461.1 MH/s for a continuous 3-hour block).
One caveat is that the popular Sapphire HD5850 Xtreme cards have less than perfect fans (cooling is great but the fans are fragile). Avoid running them at 100% 24-7 and you should be golden.
Arguably, with the difficult and market rate where they are, this is a fairly good time to buy. If there are other reasons driving you to mine then you could do much worse than putting a single 5850 in an already 24-7 machine.