That motherboard will run a Q6600 without any issues, so that would be a nice upgrade to make (2.13Ghz Dual core to 2.4GHz Quad core with more cache). Just make sure that you get a
G0 stepping Q6600 (spec code: SLACR) as they use less power, are cooler and overclock much better than the B3 stepping (spec code:SL9UM).
However, it would be a good idea to have a go at overclocking your E6400 before spending the £80 on a Q6600. To do this you will want a decent air cooler (
this one would be great and also work fine with a Q6600) and hunt down an overclocking guide for your motherboard. May I ask exactly what RAM you are currently running?
As for what you need to do to overclock- you basically increase the FSB clockspeed above stock (266MHz or 1066MHz Quad pumped). With a fixed CPU multiplier (maximum with the E6400 is 8x) this will increase the CPU clockspeed. However, to maintain stability you will need to increase the voltage provided to the CPU and the northbridge. You will also need to vary the memory divider so that the RAM frequency does not increase with the CPU frequency. The majority of the process is trial and error; you increase the settings in the BIOS, load up windows, run a stress test (such as prime95 or Intel Burn Test), if the system is stable then keep going, if the system is unstable then go back into the BIOS and increase the voltage and check whether it is now stable. If you repeat this and keep going up in small steps you will eventually reach a maximum load temperature which you don't want to go above (limited by the cooling), hit an FSB frequency which is not stable no matter what voltage used (either CPU or motherboard limited, testing with a new CPU or board would be required to determine which) or you hit a voltage level which you don't want to go over.