I have used this great app on all of the AMD 64 X2 machines that I've built over the years for various people because it is able to work like Cool'n'Quiet on steroids.

As an example, here's what I've done on my PC...

Using a fixed 35% overclock on the HT bus I have set up CrystalCPUID so that under partial load the PC uses a multipler of 9 and runs just above stock speed (which would be 2.2GHz on this rather elderly dual core). In addition I also have two further multipler settings configured: One kicks in under full load and provides a 35% overclock (so 2.9GHz in this case) along with a small over-volt, while the other setting automatically switches in at idle to drop down to 1.8GHz and a small undervolt (BTW @1.8GHz there is no detectable sluggishness while typing away at this comment for instance, so don't be afraid to give underclocking a try).

That gives me the max speed possible from my cpu whenever required while still having a nice quiet idle (so no noisy fans kicking in when watching video or browsing the web) along with the knowledge that I drop more than 40w of power consumption when idling.

The direct alternative to this is RMClock, which I have also used but found to be far more complicated to set up and also less stable (things like resuming from standby were problems). If RMClock is working fine for you then I'd leave well alone, otherwise if you have an overclocked PC this is a very easy way to get cool'n'quite working reliably.