I'd get a Duron. I have a Duron and I use it for gaming (yeah yeah yeah laugh all you want) and it works for me. I've had some experience with the celerons and I don't like them. Systems built with them just don't perform, and if this computer is going to be on the internet, another thing come to play. The way I see 99 percent of computers, they all have MSN messenger, Yahoo messenger, ICQ and AOL Instant Messenger installed all at the same time, with webshots and a virus scanner. When you start having all of that, you can't afford to have a lesser processor.
The Duron is indisputable about it's superiority over the Celeron, and its cheaper daboot.
Recommendation (if you don't plan on upgrading):
KT266 Series motherboard
Duron 1300 (only 39 bucks new online at newegg.com)
GeForce4 MX-440 (since I presume it isn't for gaming, this will do fine)
Hercules Muse 5.1 DVD (Should be about 30 bucks)
at least 256 megs of RAM
My Duron 1300 eats P4's for dinner. Oh, the Celerons make a nice snack too.