It would be more depending on you mostly play single player games or gaming online. For single player games (racing, FPS etc) your current CPU would probably be fast enough to scale with the 7950 may be up to 80% or abov, but if you play games online with lots of players, your CPU will most likely lag behind and not adaquate to scaling with even getting half of 50-60% of the full potential of 7950 (and that's not even taking into the account of overclocking the 7950).
Even your 5850 would be bottlenecked by your current CPU in CPU damanding games, particular in games that uses less than 4 cores...so I don't think it is a good idea to get a 7950 and pair with it.
On online games that I play, I use get to as low as 20~25fps in some busy areas on my old Q6600 at 3.6GHz and GPU usage of my 5850 fell to hell (down to like 50-60%), but after upgrading to i5 2500K at 4.5GHz, I now get higher GPU usage with with constant 58fps+. Had I upgraded my 5850 instead of CPU/platform, I would still be stucked on 20-25fps despite with a faster graphic card.
So what I would recommend is that you should have a good think about what type of games you will be playing the most, before deciding if upgrading the graphic card, or the CPU/platform would give you more consistant performance boost for the games you play.
