Actually, the E4500 is better than both the E6420 and the E6550.
The E6420 is way overpriced at this point -> $199 at NewEgg.
You will have a hard time finding it anymore.
The E6550 will not OC that well due to the High Default FSB and low Default clock Speed. It will underperform the E4500 when they are both OC'd due to the FSB walls it will likely hit. And that is also $45 more to get less performance.
The E6750 is a nice CPU, but will be outdated in that price range in 2 weeks, so it does not make alot of sense to build around that system at this point.
The E8400 should be about the same price, run faster at stock, OC more, run cooler, and be faster Clock4Clock.