Number of stream processors units (SPUs) is usually a good guide.
thanks anything else i can use?
Yes, don't forget to look at the max TDP Watts, it's a good guide on which is most power efficient.
All take power from the Motherboard = 75W
6-Pin PCI-E power connector = 75W
8-Pin PCI-E power connector = 150W
HD7850 = 1024SPUs = 150W max = 6.83SPUs/W
HD7970 = 2048SPUs = 375W max = 5.46SPUs/W
Therefor you should find 2x HD7850s give the same hashing power for 25% less watts.