I went back through the archives and looked at some old benchmarks from when I had a 2.4GHz Core 2 Quad 6600 and a GTX 560. There was definitely some bottlenecking going on, at least with 3DMark Vantage. Overclocking the CPU to 3.0GHz helped some, but it wasn't stable, so I had to back off. You may have better luck.
EDIT: Found some more numbers. I also had a GTX 670 and the Q6600 for a short while. My graphics scores DOUBLED when I upgraded the CPU to an i5-3570k. So yeah, you'll see some bottlenecking.