You woun't want to be changing the CPU multi with an A64! They are locked upwards, so can only go down.
Download CPU-Z, Prime95 and SuperPi. Stress test with the second two, CPU-Z give you info on your OC.
For you ...
Here ... 21st pic
... disable cool'n'quiet and set the Ht Mutli to 4x (800MHz).
Here ... 2nd pic ... change the memclock value to 166.
... 1st pic ... Raise the CPU frequency. Stress test. If you get instability, raise the processor voltage (same pic) very slightly and test again.
If you get the CPU frequency over 250, change the HT Multi to 3x. If in CPU-Z, the memory frequency gets to 215-220 ish, lower the memclock value again.
That should be it. Enjoy
// EDIT // Obviously, watch the temperatures when stress testing, especially after boosting the voltage. Lower the OC if things are getting too hot.