mobo - cpu identification and compatibility

I have been playing and working on computers for about five years but
never swapped cpu or mobos.

I am working with several older computers P1 - p2 and want to learn
how I can maximize the mobo's capabilities by switching in a new cpu
and then finding a cheap mobo to throw the extra cpu into.

Can you recommend sites that will help me to "understand"
compatibility between cpu's and mobo's. I would need information
about sockets (i.e. socket 5, socket 7) and anything else important.

Please reply to post.
Val