Consider that adding a 2nd 7970 is going to give you nearly 80% improvement, but adding a 2nd 590 is going to be a lot less reliable and probably will only add about 50-60% improvements. Crossfire will scale better than 4 way SLI.
Yeah true, 7970's are quite abit cheaper aswell. Thing im scared of is if I buy them, then a new series of cards come out. But should be ok because the 7000 series from ATI came out not long ago.