Those are really cheap 1GB cards and the 2GB card you are looking at is one of the newest models. Not only are you paying a premium for being the latest and greatest but you are also paying for the DIRT 3 name. Unless you have a huge crush on that game I would stay away from the name recognition. Also, sapphire already has a non-reference model out and I would be curious to see if this model beats the other one in cooling since that is what they are designed to do.
On a different note, I think you may be a little confused as to what the difference between the 1GB and 2GB models is.
The ONLY difference between stock 1GB and 2GB model of the HD6950 cards is that the 1GB card has 1GB of GDDR5 ram and the 2GB card has 2GB of GDDR5 ram. Otherwise the cards have exactly the same specs.
The amount of ram on a video card comes into play in a few notable situations. Here is a link to albeit an older article but still relevant on the topic of ram for a video card.http://www.tomshardware.com/reviews/graphics-ram-4870,2428.html