I'm running a GTX570 with my Q6700 oc'd to 3.2GHZ. I've estimated my bottleneck to be between 5% and 10% depending on the game. I must have run a dozen benchmarks. The bottleneck isn't really even noticeable, save for a few hundred futuremark points.
Your Q9400 should meet or exceed this overclock, depending on your cpu's vid and your cooling budget. That being said, an I5/I7 has architecture and efficency improvements that speed up most computer tasks. My 3dmark scores may be in the same ballpark as a similar spec'd machine with an I5, but my Linpack results are not in the same universe.