The Sempron 64 2800 754s seem to overclock well. I have one on a Gigabyte board running at 2.0ghz from 1.6 with no voltage increase. Just lowerd the HT to 3, dropped the ram to 333 and raised the FSB up to 250.
i also have a athlon 2500+ on a socket A( ) which apparently will overclock very nicely too, and the great thing about the sempron one as well is that if i do cook it, then they're very cheap to replace(plus a good excuse to upgrade to AM2 instead )
SOCKET A WAS GOOD IN ITS DAY, BUT AM2 IS FOR ME AND YOU