Im sure you can get at least 3.7GHz with the E8400. ThatGuy16 has 4GHz, I know water cooling but he had it at like 3.8 or 3.9 on air.
Now if you really want a Quad, why not 45nm? The Q9300 runs cooler than Q6600 and E8400 sometimes. Overclocks very stable @ 3.0GHz and some people push it even further to like 3.5GHz I've seen in forums...