What is your monitor resolution? It seems like a worthy upgrade, but I don't know if its worth $200-that's up to you. Your question is a fair one, it might behoove you to have 3 or more cores available. You won't be bottlenecked as far as clock speed goes however.
Have you tried to unlock this CPU? How about overclocking? I wouldn't rush out and buy a new CPU, there are things you can do to get a little more performance outta the X2. I would try to unlock a core or two, and check for stability.
An X2 at 3.4Ghz can outperform a quad core at 3.0, but it is application dependent.
Some games use more cores than others, I believe left for dead and maybe dirt2 are well threaded. Other games might use two cores really well.
Overall, I don't think you need to worry too much about bottlenecking your new GPU. I think you could set your multi to 17X or 18X without any voltage and be in good shape.