Personally, I've had mucho quads...and now I'm back to dualies because they just seem faster for basically everything except encoding and rendering.
4.5gHz 45nm dualie owns all
Also despite what many people may think, my dualie actually handles Solidworks and CAD better than my quad setups did (except the 4.2ghz QX9650, but that was too expensive to keep).
I still want a quad, but only a 45nm E0 stepping which I won't be able to afford for awhile. Anything under 4.1-4.2ghz will leave me sorely disappointed.