It depends a lot on your preferences. A HD 5770 crossfire will produce good results, on par with a 6870 or so, and might take a few victories from a HD 6950 1GB if overclocked. That said, power consumption at idle will increase a bit, and noise at load will increase a lot. I had CFX 5750s, and while I didn't personally notice any microstuttering, that too can be a problem, depending on how good your eyes are and how much perfection you demand. All in all, you're still going to be CPU bottlenecked in the titles where you currently are, and the second card will only improve your GPU-bottlenecked titles.
So, improve your balance? Not really, you've got a reasonably nimble system as is. I'd only make that upgrade if you must have better GPU performance. Resale of the 5700 series of card is terrible, so I have a really hard time recommending putting money in a new one...but, if a refurb comes up, then it might be worth it.
Otherwise, wait for Southern Islands or Kepler based products.