I always built my own machines, except for my first Pentium (which ran BeOS prettt well!)
I was using an Abit motherboard back in the P3 days. I'm not sure exactly which one... maybe the VL6? I remember the VP6 and BP6 were popular because they supported dual CPUs. I don't remembered my P4 motherboard. It was probably also an Abit.
Edit: Fortunately, Newegg still has my order history from back then. My P4 motherboard was an Asus P5GPL