Full details of the MOB can be found here
AOPEN a downloadeable manual
here
If you are in the UK, some places to look are
DABS Planetmicro Microdirect Novatech
It supports P4 CPU, AGP 8X, DDR 400 [PC3200] memory 2 slots, onboard sound, USB2, ATA133.
Not sure what the spec is of you original PC but you may well be better off buying a new case with sufficient PSU, approx £35, CPU and Memory. I recently did an upgrade on the kids pc although with an AMD 2800+ new case new mob and memory and it was around £200. You should be able to use all the peripherals that came with it i.e. floppy, HDD, CDROM cables etc.
You may well have to start afresh with the operating system as it doesn't always like big changes like this so back up all your important documents etc.