Most of the MP boards I have seen have 2 64bit slots and the remaining (3?) are PCI... but I'm sure you've noticed that yourself... 32bit PCI cards should work in a 64bit slot, at least on the servers I've worked on (which had all 64bit slots).
AGP Pro is used by the fancier 'workstation class' videocards... It was my understanding that it's basically an extra connector at the front of the slot that allows more power to be delivered to the videocard... so you could even have an AGP8x AGP Pro slot, but all I've seen so far is AGP4x
Regardless.. here's the spec:
http://www.agpforum.org/downloads/apro_r11a.pdf
When picking Processors for an MP system... don't rule out the 2500XPs (since they're barton too) and considerably cheaper than the 2800s... btw, you do know about the
L5 'mod' right?