Alright, get the Athlon XP, but I will add this one last comment. If you are going to get an Athlon XP, then the XP-M should be your choice because it is the better overclocker and you should try overclocking. Look at Nubius, he has his 2400+ running at 2.7 GHz and that is great.
Oh, the Athlon XP isn't a very good processor, at least the Barton aren't since they don't match up to their performance ratings. The TBreds and older XPs do. The 2700+ will outperform a Celeron D, but I still will not recommend because I value upgradability. I buy a computer once every three or four years. In the meantime I upgrade as my needs require. For example, take my Pentium 4 Socket 423, I upgraded that to the max and I had 1 GB RDRAM in there and a GeForce FX 5950. If I would've bought a new system every year , I wouldn't be talking to you right now.
Contact Nubius, he will be able to help you better with AMD XPs since he is more educated in AMD than I am and most of the people who use AMDs are using the Athlon 64s not the XPs.