Here is some Speed Review comparing HD5870 and HD7970.
since this 2 Cards is use single core GPU, not like 6970 or 6990.
I test use the same system (same komputer) Win75 64Bit with oclHascat-0.09 and Catalyst 12-1
HD 5870 in Stock Clock 850Mhz reach about 20xx Mhz/s 
HD 7970 in Core Clock 850Mhz reach about 25xx Mhz/s 
HD 7970 in Stock Clock 925Mhz reach about 28xx Mhz/s 
HD 7970 in Core Clock 1000Mhz reach about 30xx Mhz/s 
Now you all do the conclusion
according to the power, absoloutely buy 7970 is worthed for Bruteforce.
but considering the price for now, you all do the math according the price in your place.
Best Regards,