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)
Ill mostly be using my system for gaming, as well as some CAD/CAM software for college. I figured the Q6600 would be good for upcoming threaded games since Supreme Commander has shown improvement when using a quad core processor.
Yep it sure has. I using a 4200+ dual core with a cruddy graphics card atm and I can say no matter what settings I use supreme commander gets laggy as hell as soon as there are 1k + units and they all start fighting.
Supposedly, with the quads and a desent grapgics card, which I have waiting atm (7950gx2) it wont slow down on medium settings no matter how many units are in play.