Well, you could put in one of these, which is a 2.4ghz 400mhz fsb:
2.4ghz Celeron, but you'd get much better performance out of one of these, if you don't mind overclocking:
2.0ghz P4A. They run up to at least 2.7ghz, occasionally higher. But if you spent $97 on an Abit IS7:
link, and $178 on a 2.8C:
link, that's nearly $300, and you'd still have to buy yourself some good PC3200 ram. This Geil works well with P4's, and it's cheap:
link, but that adds another $127. You really can't build an Intel system for less than $300, if you include motherboard, processor, and ram.