A couple of thoughts:
1) if you're not that intent on performance, also consider a non-XP Athlon (the Thunderbird) and you'll have more motherboard options.
2) if you can find an Asus A7V-133, the 1.05 revision supports AthlonXP's up to 2100+ (Palomino core only, I presume, not Thoroughbred) with the proper BIOS. If you got one that was pre 1.05 then it might still work, at least one person had success that I recall. If it didn't have the necessary BIOS revision already then you'd have a chicken-and-egg situation. There should be other similar boards from Shuttle, EPoX, Gigabyte, etc... just do your homework and root around on their sites.
The ECS K7S5A would be a good all-around candidate if you could live with two memory slots. RAM's inexpensive enough that you might just pick up a new PC133 module to go with this board. Remember that an AthlonXP is going to demand more from your PSU, and it's not worth the loss of hair to try scraping by with the PSU that ran your Celeron300 just fine.
