E6400 is, by far the better choice for overclocking. Depending on the particular chip you have, a 50% overclock may be possible. A 30% overclock is likely.
My first Core2 system used a 2.4 GHz E6600. I got that up to 3.6 GHz. But a lot depends on both cooling and case choice.
Alright So Around 2.6 to 3ghz is possible on a stock cooler? Well its not stock, its a cooler master CPU Cooler (X Dream 4) and they claim its around 10% better than the intel stock cooler so yeah.