I would like to comment on a setup that I have as a comparison.

Athlon X2-6000 (dual 3ghz) and a Phenom 9850 (Quad 2.5Ghz) on the same mobo (Asus M2N-SLI) with Nvidia chipset running at 1.5Ghz Hypertransport

Ram is irrelevant.

Single thread apps work slower on the quad core.
Multi-threaded apps (Like video encoding DivX) operate faster on the 9850 by a margin of about 37% faster than using the X2-6000.

Noticeable difference? No.

If I wanted a noticeable difference, I would need to upgrade my Mobo to a AM3 chipset with at least a 3ghz clock to notice a significant margin of speed.

I understand your experience with using a tripple core (Why AMD sell these is beyond me)