A Q6600 should still be bottle-necking the 4890. From my experience, a Q9550 is the closest you can get to matching the Radeon 4890 in performance when it comes to 3dMark benchmarking.
If I were you, I would play around a little with overclocking first (ONLY if you are comfortable with overclocking).
If you are comfortable, OC the video card a small amount and see how much the performance increase is.
Set the video card back to default settings then OC the CPU next. Run 3dMark again and look to see the performance increase.
If the video card OC improved the 3dMark score more, then a second video card would do you good. If it was the other way around, plan on overclocking your CPU quite a bit if you still plan on running x-Fire.