A 590 is much faster than a single 7970.
Both can OC, so the difference will remain around the same. It's not fair to compare them, regardless. One is on 40nm, one is 28nm. Throw on top that it's a year old card...and yeah, real impressive. The 7970 is an ice card, but is on average 10-15% faster than a 580. OC'ed it can be about 30%, tops. A 590 is two under-clocked 580's, which can be OC'ed further, bringing more performance than a 7970 by about 50%.