Guys like you should be reading reviews more often. If you are upgrading it shouldn't hurt to invest a half hour to read some reviews to get the picture which CPU performs better. Not only that, you would get some basic understanding for which CPU + motherboard combo you should use.
Anyway, Athlon XP 1800.
The 1.4 runs mighty hot. I've got one, and the additional cost of getting a silent and yet powerfull enough HSF (swiftech+pabst for me) will cost you more than an XP1600 or 1700 that run considerably cooler. Also, the SSE support might come in very handy, depending on what apps you run. I'd go for an XP either way. Pick your clockspeed depending on your budget.
= The views stated herein are my personal views, and not necessarily the views of my wife. =