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 ?
7970 is going to be 200 watts I believe, and the 5870 is 188 (both at stock watts). This is where the 7970 suddenly shines. Even if the 7970 is 250 watts, thats still a jump in efficiency.