hbriggs :
What is a reference card?
When a chip maker (nVidia or AMD) make a new GPU, they give third parties a "template" we usually call "reference card". So, when they release the cards, they usually carry the good and bad choice calls from the original design. Down the road, 3rd parties usually improve the original design (ref. card) in some areas where they see fit.
In this particular case, the Ref card (template) from the 6950 2GB series had some things "locked", so you could "unlock" them to be a full 6970 2GB.
Now, to the original topic: 2x6870's in rebate or offer are unbeatable. I'm currently using a 6870 from a friend (got 2 from rebate for like 140usd each) and it's slightly better than my 4890 in DX9 at 1080p. I can tell the difference though, plus it handles 1080p quite well in BC2, so BC3 should be enough for a single 6950 1GB even IMO (that's DX11 I'm talking about too).
Cheers!
EDIT: Typos and added idea