Actually...
I don't really know what the SocketA semprons are like, but the Sempron 3100+ are really good for overclocking. And it's just kinda like getting something for nothing - however fast your processor, you can always make it that bit faster. But you would, still, be better off with a similarly priced AthlonXP and overclocking that I think.
And you should use a program called Prime95 to test if your overclock is actually stable.