In the UK currently :

5870 costs 170 GBP and gets 440 mhash/s so about 2.6 mhash/GBP

7970 will cost rougly 430 GBP and get 666 mhash/s so about 1.6 mhash/GBP

Thus, the 5870 is still much better and you can also get a 5970 that gets 850 mhash/s for about 400 GBP.

Power figures ?