Currently the Phenom II X6 doesnt work very well with linux. The cpufreq-driver misinterprets the frequency stepping of amd processors with turbo core. Therefore the CPU cannot run at its full speed.
This can be solved with a kernel patch or with deactivating Cool'n'Quiet. Future kernels (e.g. 2.6.35 and 2.6.34) will solve this. The date when this patch is included in 2.6.32.x, 2.6.33.y is unknown.