7970 will give you along time between the next upgrade.
No such thing as overkill.
For an single 1080p monitor, it is Overkill unless if you want to turn Tesselation to x64 and the AA to x8 with everything else at Ultra.
The HD 7770 is serving me very well right now to play BF3, at High settings, 1080p no AA. I get approx 68fps.
Demanding games will take advantage of that card and you still might not always max everything out and get over 50 FPS. I personally would find it perfect except for the price. Of course I also use a 120hz 1080p monitor.