Ike, the answer to that question depends on what you plan on doing with your system.
For gaming and office applications the Athlon XP and Pentium 4 are very close to each other. The Athlon 64 is a cut above the Athlon XP and Pentium 4 when it comes to gaming.
For video editing applications the Pentium 4 is typically faster than the Athlon XP. The Athlon 64 is fairly even with the P4 here.
For engineering/scientific applications, the Athlon XP and Athlon 64 are generally faster than the P4.
There's also the issue of cost/performance. Neither the Athlon 64 or the higher end Pentium 4s can rival the performance per dollar offered by the mid-range Athlon XPs.
It's also worth considering that both AMD and Intel are planning new processor releases in early February. This means upgradability on current motherboards is not very good.
My recommendation at this time would be to wait if possible to build a new system until March or April. By that time the next generation Pentium 4s will be available and the Athlon 64 will have dropped in price to the point where it is affordable. (There are also very few motherboard choices for the Athlon 64 at this time.)
If you can't wait and are looking for the best value that would without a doubt be either a Pentium 4 2.4 or 2.6 'C' type or an Athlon XP 2500+.