I think it's not really a NB, this is just the IMC speed. On K8 it was always 1:1 with the CPU, apparantly now it's seperate.

This seems interesting to me as on K8 the IMC was suspected to play a major role in the max CPU clockspeed. A typical good K8 had mem revision B1 or BB as you will know, which would get you higher than a good week BW.