For me getting the 1280MB is fine and oh course somewhat this is not worth it and then this is my opinion. Yours might be different. I will get 1280MB with aftermarket nice and good cooler if I have the money and I will not buy the 2560MB because it's a better way to safe money. Or I can just get nice 6970 (with aftermarket cooler of course) and OC it, it will maybe outperform GTX 580.
The added memory will only kick in at extreme high resolution and high details. For normal single monitor gaming @1080P the std 1280Mb is going to perform just as well.