You can indeed match a 7970 with a 7950, check out the compatibility matrix here
http://www.wsgf.org/article/amd-crossfi ... ity-matrixAs for the 7950, it will of cause add some additional performance, however what puts me off the 7950 at present is all these recent talks of FPS latency and microstuttering, where by the 7950 has been picked on for being quite bad, its really been getting a grilling these last couple of weeks.
A few weeks ago I'd have said sure go for it, but now I would hold fire, do some more research and see how this might effect you.
You may see your games running at a higher FPS, but not actually running any smoother due to latency introduced by the 7950.
Maybe, if you ask nicely
ask skipclarke to perhaps do a fps and latency graph for a game or two across the 2 cards alone and then them combined.
He is a busy man so I hold no guarantee but worth politely asking