Don't really know

There are a lot of things to consider, first, the fan speed and the fin height, double the height and you can cut the speed in half. Then there is the buffering capacity of the heatsink. With my 2" copper pipe-plug I was happy as a pig in sh*t with the P3, lowest temps ever but it wouldn't work with the Athlon because temp spikes were just too high.
8 mm aluminum base are the minimum recommendations right now (0.6 mm what I was using with the copper plug), if you use a countercurrent scheme, you have to be aware that this might be the worst case scenario because that is what was designed by duck feet to overcome temp gradients of 38 centigrades or more during the winter.

I am still building my own heatsinks, though, but it is a learning experience.