Check out the New BFG 512 mb 6800 Ultras.
Unfortuately, it can't be bought anywhere except here:
http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=326014
And its out of stock. DAM.
You can also buy it as part of an alienware computer.
http://www.alienware.com/intro_pages/6800ultra_512.aspx
But I'm not paying for their insanely overpriced computers. I'll wait for zipzoom to get some in stock.
Can't wait to get two in SLI on my comp.
According to nvidia, I should see a 30-40% increase. I seriously doubt that as the latency of the extra 256 mb would hamper performence, but I'll see how it will turn out.