I came from an overclocked 6870 to an overclocked 7970, I game at 1080 and noticed an enormous improvement. Can play BF3 at ultra settings vsynced at 60fps, and it has better graphics than real life
A 1gb 6870 would have had Vram issues in BF3 at ultra settings. A 2GB 6970 manages it at 1080p ultra fine.
I'm not saying it's not better but if you have a 6970 and game at 1080p a 7970 is not going to give you a £500 boost of performance. If you are using say 2560x1440 or eyefinity setups the 7970 is superb.