Don't consider anything except an E5200, e5300 or e5400. They've great overclockers, have high multipliers, are based on 45nm wolfdale tech, and they're cheap as he'll. I built a system for a friend with one and it's so good it's making my q6600 jealous.
If an E5400 is making your Q6600 jealous, your Q6600 has issues.
Q6600 > E5xxx any day of the week.