Change/upgrade PC motherboard

I have two old PCs. One is IBM Aptiva PII 400 MHZ. Another is Toshiba M500 PIII 500 MHZ. Can I Change/upgrade PC motherboard using P4 or AMD chip for them? If I can, please tell me what kind of motherboard I need for each of them. If you think it is too hard or take a lot of time, please tell me as well. I'll buy a new one.

What kind of CPU do you recommend between P4 and AMD?

Thanks a lot.