If you intend to keep your Q6600 then the only benefit is you coould aim for higher bus speeds.. but we are talking extreme speeds. DDR3 effectively has a better throughput so youl could run the RAM at a higher frequency but this won't improve performance by much.
I ran both DDR2 and DDR3 on my old P5KC motherboard and benchmarks should little or no difference. However, i always suspected a flaky controller on this board so who knows. Bottom line.. stickto what you have for the moment and upgrade the whole lot when you can.