I'm currently using the same cpu as yours, amd 64 3000+ socket 939 at default speed (1.8ghz).
I used to overclocked it to 2.4ghz (like 3 or 4 years ago). It was running at that speed for around a year or so until I noticed my computer kept getting restarted by itself (apparently due to summer heat). Since then I run the default speed again until now.
If you need to overclock, all you do is increase the FSB in the bios, may need to adjust the voltage as you increase the FSB.
Fyi, for whatever reason I can't seems to overclock anymore. It just failed to boot, but fine with default speed.