They can be had for around $275, and they are unlockable to 6970s with a simple bios flash.
That easily gives it 375 MHash on stock clocks, and definitely 400+ with some OCing
I read a few places that flashing the 6950 with the 6970 bios is not good for the card, but there is a mod'd 6950 bios that will unlock the extra Shaders. Made my 6950 jump from about 330 MH/s to 370 MH/s at an 870/625 OC. Pretty sure I could crank it higher, but it's my main gaming machine, so I don't want to blow the card.
If you want to risk it and jump to the 6970 BIOS, Techpowerup has a good article on it.
If you just want to unlock the shaders, check out the following post.
Regardless, fudging around with the bios is risky and you could brick your card.
Though I still don't know if the 6950 at $275 for maybe 375 MH/s still isn't worth it compared to a 5830 at ~$100 and 270 MH/sec.