Sounds very much as though you have an old AT (as compared to ATX) motherboard and more importantly, power supply (do you have a case?).
This limits you to this type of equipment, which ended with the very early Pentiums.
You may be better off looking on e-bay for a complete base unit that is of the later ATX type, or at least a shell (case, motherboard and CPU - memory if possible) and then add the peripherals to that. You can pick up a brand new keyboard for under a fiver!