not enough
Processor and motherboard is not enough.. the thing that Apple has done to prevent clones.. and been succesful.. is require the ROM Chips to be present as well. This is what you will have trouble finding. This is also why there are no Mac emulators around, other than the fact that a CISC processor will have a hard time emulating a RISCbased processor, i.e. powerpc..
You can certainly buy used boards online that have the ROM chips with them, but I don't really see the point.. You won't be able to 'out-do' Apple. If the processor for a faster Mac were available, then you would see it at store.apple.com, but you don't.. you can get G3 and low-end G4 chips/boards, but it's gonna be pretty pricy..
Just buy from Apple, or a complete used sys..