I agree with ahsia, but I wouldn't necessarily throw overclocking into the mix so quickly. Many people recommend products based on their overclockability, but that's not something that you should do unless and until you're very comfortable with the technology and know exactly what could (and will) go wrong. I mean, people used to recommend the old Celeron 266 chips because they could overclock so well, even though that both before and after overclcoking they sucked, and there were many other issues to think about.
In terms of your situation, it's up to you, depending on the types on deals you can find. I'm assuming that the rest of your system is OK, but since that doesn't affect the core question, here goes.
Athlon XP and Pentium 4 are very similar in peformance, but Athlon XPs are slightly faster in both synthetic and real-world benchmarks, but not noticeable so. I have a P4 1.6 Ghz at home and an Athlon XP 1800+ here at school and I can't notice a difference in just OS usability. An Intel-based system is--and yes, I say this out of experience--likely to be slightly more stable thanks to a better engineering process on the part of Intel vs. VIA. VIA has gotten and is getting better all the time, but my VIA-based system is slightly flakier than my Dell. Maybe it's just my MB. Athlon XPs cost less, too, so you'll save some cash there and you can upgrade another part, too. You have to consider cooling with an Athlon XP, though, and you don't at all with the P4. Let's review:
Athlon XP
Pros:
Fast
Cheap
Cons:
Cooling issues
Greater dependence on quality hardware to maintain system stability (just shop around, find a good MB and you should be OK)
Pentium 4:
Fast
Stable as a rock (again dependent on hardware, but better manufacturing process with Intel, likely)
No cooling issues
Cons:
Expensive
Slower than Athlon XPs
If you decide to go with the Athlon platform, wait until the end of the month and into next month for the release of AMD's Thoroughbred-core CPU, which with have a much easier time cooling since it's on a .13 micron process.
It' really up to you. Weigh the costs and do some math, shop around, that sort of thing. I prefer AMD to support the underdog, but it really comes down to personal preference.