I've written a Linux utility to set the CPU clock multiplier on K6-III+/K6-2+ processors. These are the processors that support AMD PowerNow(tm).
Download it from http://graflex.org/linux/k6mult
I have currently tested it only in RedHat 7.1, but it ought to work in any reasonably modern system 2.2.18 or 2.4.x kernel.
The utility includes a command-line program to set the multiplier immediately, and a boot-time service to set it on boot.