Have you actually calculated that out?
You do understand that a higher hash rate will compensate for more power draw?
No, I figured more in comparison to, say, a 280. probably not a 290 or other, yes.
I am not an authority on this, that's why I am asking.
A 7950 is good for 200-250W. You could probably buy a 280x, under volt/under clock it, and still get the same hashrate and power draw. All from a newer card that (might?) costs less up front and will likely be worth more down the road.
There is no authority, and you really need to figure this stuff up on your own! I've no idea what prices are like in your area for electricity and gpus!