If it is a socket A (Thunderbird) 900 Mhz processor it's going to be a 100 Mhz FSB type. If I were buying a board to support it, I would get a good new one. That way you could use at least up to the newly released Athlon XP 2100+.
By the way, you don't have to get a board that uses VIA drivers. I had a good experience with the MSI (microstar) K7N420Pro, which uses the nVidia nForce 420 chipset. (Windows 2000 or XP highly recommended.) If you don't have a GF2 Mx or higher video card, this board has one built on, along with very good integrated sound.
There are many good boards available using VIA's KT266a chipset as well.
(Both use DDR SDRAM by the way. I highly recommend a DDR SDRAM board for top performance and upgrade reasons.)
Building your own system is not difficult, but I do recommend seeking help mounting the heatsink on the Athlon if you've never done one before. (Maybe your friend ... ) You need to be pretty careful when you do it to be sure the heatsink is flush on the die and oriented correctly before applying pressure to the locking clip. It's not hard if you've done it before.