I am running an Athlon XP 2000+ on my current machine (until tax returns... then E8400), and my brother is running an Athlon XP 2400+ in his computer. I have tried overclocking both of them, and they are both horrible. I was able to get my FSB up to 150 with a slight voltage boost, but it was hot (45 C at rest, 70 C with Orthos), and not 100% stable. 100% stable was at something like FSB 140. My brothers was about the same.
The best bet is to see what the fastest CPU your motherboard can support, and search eBay. You can find Athlon XP 3000+ for around $30.
A friend of mine is giving me his 3200+ for free because he is getting a new PC, I guess I will have to keep this computer for my wife.