The performance dividend on AMD systems isn't worth it to upgrade to higher clocked memory because the performance difference just isn't that high. 5-10% is what I've seen from those kinds of modules.
Some great examples:
and here are some DDR3 speed and latency tests:
It's just not worth it for such a small performance gain to spend so much more on memory that isn't performing all that much higher.
I mentioned that I run Mushkin black line in my system at home, but I didn't mention that I run with all of my slots full, so my DDR2-1066 modules only run at DDR2-800 speeds (CPU limitation), but the performance loss, even accounting for the drop in size (when I tested the speed difference, I only had 4 of the 8 GB installed, which was two dimms) the performance gain was only 4% on each test, at the most. AMD's DDR3 memory controller is no different.
It's not that AMD's memory controller sucks, it's just that their architecture just doesn't NEED the insane memory bandwidth requirements that Intel's i-series CPUs does.