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Hi

The 6950 and 6970 are the same, the 6950 has just some shaders unactivated.

If you want to activat them you have to flash the card

http://www.techpowerup.com/articles/overclocking/159

This was known within the week the 6950 was released.

You're telling us nothing new.

Can somebody suggest which 6950 would be best to buy?  Do all of them unlock or are there certain ones that don't?

If you goto newegg and search '6950' it comes up with something like 23 results!!!  All appear to be in stock.  Prices ranging from $200 after MIR to $300.  Only difference I can see is that some have varying OC settings from factory.

Do not buy a 6950 relying on being able to unlock the shaders.  This has been beaten to death over and over on other threads here and several other OC forums.  

There is no 100% guarantee you'll be able to unlock shaders on any particular card, even two cards in the same production run may act differently, and the odds of getting one that actually will unlock seem to be dropping quickly.

Avoid the 1GB models if you want to have a chance; none of these are reference cards, and from my personal experience and reading accounts of others, it's highly unlikely you'll unlock shaders on these models.

The 2GB cards have better luck, if you find a reference 2GB 6950 it's your best chance to unlock shaders.  However, newer revisions of several of the 2GB cards appear to be hard-locked (physical disconnect of extra shaders).

Read through the reviews for a particular card to get an idea if it seems to be unlockable.   Also assume that you'll be voiding your warranty right out of the box, not every company is as lax as XFX ;)

Addendum to the above since I didn't actually answer your question - I have 4x XFX 1GB 6950s (ZDDC model @ 830mhz with two fans).  I can push them to about 355MH/s.

In linux, I cannot downclock the memory more than 125mhz less than the GPU clock, I can't overclock any of them past 885mhz without issues, and I can't alter the voltages to try to stabilize them past that.  Not that I would want to - they run hot, and suck down near 200W each on stock voltages.

I can safely say I won't be getting more 6950s unless there are not other reasonable options.  Without the 58XX series cards in the mix, however, they're pretty much it if you want a semi-reasonable $ / MH ratio.

I got 2 6950 2GB sapphire reference design recently, 1 new 1 used, both successfully unlocked 1536 shaders, need to do some command line to unlock the bios first(write protected by default), and I've read that only reference version has high % of unlock.

Both can go to 900mhz core at stock voltage, full stable, they get ~400mh/s  each

Only reference cards can be modded.  NO NEW CARDS CAN BE MODDED.  None.  Don't believe what people will tell you.  Only the first ones that came out that were reference cards had the dual bios and had the shaders that could be unlocked.

This.

Don't buy the new cards, especially 1GB, unless you want to mess around with a 'bubblegum hardmod' which might not even work.

If you want a 6970 for the price of a 6950 through BIOS flashing, try finding one of the old reference models.

Actually the 6950s don't do very well in terms of Mh/$ ratio. You said yours max out around 355Mh/s. Newegg lists the XFX 6950 1gb model, which is the card you mentioned, at $240, making the Mh/$ ratio just under 1.5 Mh/$. That being said, there are plenty of cards available now which easily clear the 2 Mh/$ mark.

The 5770/6770 cards (essentially the same GPU) do 220Mh/s and above, so if you can buy one for under $110 (which there are plenty of deals for $110 and less) you've broken the 2 Mh/$ mark, assuming you can push it past the hash rate I mentioned.

5830s can still be found, although they are becoming increasingly difficult to find. That being said I bought two a couple of weeks ago for $130 each and I have them doing 315 Mh/s for a ratio of about 2.4 Mh/$. This is about as high of a ratio as is currently possible with available cards.

5850s are all but gone from the marketplace, however I used to see these sell for $160 and it's possible to push them above 400Mh/s, making its ratio a solid 2.5 Mh/$. If you can find one, this is about the best you can do in terms of Mh/$.

2 Mh/$ seems to be the mark that everybody aims for, and there are plenty of cards that can do this. As for the 6950, the best bet for maxing out its Mh/$ ratio is hoping you can get a reference card whose shaders are able to be unlocked. This adds a significant performance boost into the 400Mh/s territory. Even so, the ratio would be under 2 Mh/$ but 400Mh/s is not something any card with a 2 Mh/$ ratio can hit. At a certain point, it becomes less important to get the highest ratio possible in favor of getting a high Mh/s rate from a single card.

Sorry ManOfKnight, but you are WRONG. I speak from personal experience, not just saying I heard it somewhere on the internet. And I'm talking brand spanking new model cards with twin fans (non-reference) and only 1gig of ram.

airdata, I suggest that you go over to a hardware modding/overclocking site and do some reading there. (http://www.overclockers.com/forums/)

Not saying it is easy like it was when the first reference cards came out, or even that all new cards can be unlocked (they can't).

EDIT:
Here is the page I use to unlock my 6950's
http://hardforum.com/showthread.php?p=1037577079

I use a CircuitWriter pen instead of the bubblegum. ;) All you have to do is write on top of the chip to connect the two pins (very easy).
http://www.amazon.com/gp/product/B0002BBVQO