Upgrade Motherboards or Buy New Box?

Being a relative novice to hardware, I’m not sure whether it would be better to buy a new Case/Motherboard/CPU and transfer the hard drive, cd-rom, network card, etc. or just put a new motherboard in the old cases (April 99). I have four workstations that are identical, and need upgrading since they are running 350 AMD K-6 motherboards. Any experiences? Suggestions?