Currently running a Q9550S with a single HD6870. Going to go add another HD6870 for crossfire. The choice here is to upgrade to a high end 775 board for cheap and rock the Q9550S or go new CPU, Motherboard and RAM to get the i52500k and Crossfire.
CPU wall for the crossfire HD6870 on the Q9550S ? Or at this level is the upgrade to i52500K a price poor alternative when it comes to performance boost v dollars ?
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.