I would not invest much more in an obsolete chipset.
Today, the 2500K will run any graphics configuration very well, and looks to be able to do so through the next generation.
Since you are going to add a second 6870 anyway, go ahead and do that first. See how you do.
Then adjust your cpu power to see how sensitive you are to cpu changes.
Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
Or, if you can, OC your cQ9550S a bit and see if your FPS improves.
My guess is, that you will find a 2500K a compelling change.