Well, the ammocrate idea is neat, but I doubt the motherboards I'd suggest will fit in it. I'm talking about the Abit NF7-S 2.0 and the Asus A7N8X. Both are great motherboards, and neither will fit in that ammocrate (my guess, anyway). You'll have to get a smaller motherboard. I'd try to find one with an nForce 2 chipset, PC3200 RAM support, and 400 MHz FSB Athlon XP support. 8X AGP is a standard these days, so that shouldn't be a problem.
If I were you, I'd buy a Shuttle and fuck the ammocrate idea. I've seen it done multiple times on Bit-Tech. It'll just put weird variables in the equation, like sizing and spacing, etc. If you fuck up one hole with a Dremel, it'll look like shit. I'm not sure if Shuttle has mobo combos for Athlon XP systems, so you're probably gonna go with Intel if you buy a Shuttle.
*EDIT* NewEgg has AMD Shuttles now. Kick ass! Go with AMD if ya get a Shuttle.
-Lasereth