This is what I would get:
Thermaltake SilentTower 4 in 1 CPU Cooler for Intel P4 478 & Socket T, AMD K7 & K8, with Heatpipe Cooling
This thing can cool an Athlon 64 and a Pentium 4 3.6GHz. Your Athlon XP is nothing compared to those. For your Athlon XP 3000+, you can OC it like no tomorrow and it will still run cool. Get this one. It's also got good reviews from Newegg, check it out.
How about just using stock heatsink? I overclocked mine to 2.4 GHz and still running at approx 27 degrees?