Yep, I had 2 Core2 chips I overclocked. An E6600 I got from 2.4Ghz to 3.4Ghz and an E8400 from 3.0Ghz to 4.050Ghz. They are very easy to overclock. You will need to start with aftermarket cooling. Do you have a good CPU cooler?
Here is a pretty much step by step guide.http://www.tomshardware.com/forum/259899-29-core-overclocking-guide
With a Core2 chip you just do the chip's multiplier x the front side bus ( FSB ).
The Q9400 has an 8x multiplier. That means at stock it has a 333FSB because 333 x 8 = 2.66. Since the multiplier can not be raised all you do is raise the front side bus.
So to overclock to say 3.0Ghz you would raise the FSB to 375.
Just read the guide and if you have any questions feel free to ask.