How much do you want to spend?
Dollar for dollar the best cards I can think of that come to mind...
Hd 5770: less than $100. Generates 200mh/s easily enough.
hd 5830: Between 100 and 200. Generates 300mh/s or more.
Hd 5870: If purchased for less than $200. Generates 360+ mh/s.
The hd 6950's are around 200 dollars after rebates from XFX, those put out about the same hashes at 5830's

Now, if like me, you find a 5850 or 5870 for less than 200, pick it up asap.

You should overclock them all. Even a bit. It gets you more for your money.