You can even crossfire a 7950 with the 7970. If you want another 7970, get the HiS or the MSI version. Those are way better than an XFX and really worth the money. You can oc those to very high clocks like 1300mHz or 1400mHz or even higher!
At what resolution do you play ? If it's 1080p it's not worth the upgrade. Crossfiring a 7970 with a 7950 would mean the 7970 will get downclocked to the 7950's performance and basically you will have a x-fire of 7950's, which again is not worth it.