Power wise you wont get that much of a gain going from 2 5870's to 1 7970, BUT the biggest advantage would be the extra Vram. The 58xx's only had 1 gig compared to the 79xx's 3 gigs. Going from my single 5850 to my 7950 doubled my performance. You have two cards in crossfire is going to negate it being as big of a jump for you. While you may see some improvement dont expect anything drastic unless you go with 2 7970's.