What I've heard the Quad's don't overclock as well as the duo's.
What are you using this system for? if you plan on using it for video rendering/editing (or anything else that makes full use of 4 cores) I have no problem suggesting a Quad-core.
But if you plan on gaming - an e4500 (overclocked plus heatsink) or e6750 (minor OC) would cost less money and likely perform better in games for another year or two. By that time the quads will be much cheaper (and better utilized in more programs/games).
So for about the same money you could get a Duo and with a higher OC, which will perform better in games now, and later - pick up a cheap Quad once the benefits are there. (this is my plan anyways, but I plan on mostly using the system for gaming)