I'd say anywhere between 600-700MHz. They tend to have the most success with overclocking. See fivespeed5's sig for a perfect example.
One thing I must warn you of however, is that all these modern AMD CPUs (Duron, TBird) run very hot. If you buy one, make absolutely certain you go overkill on cooling. Otherwise you'll be running at insane temperatures and if you try to overclock, you'll toast it.
Best of luck.
Cretin