I'm assuming you mean a blown on-board NIC. In which case, if the board still boots and runs, and just the NIC doesn't work, i'd run a PCI add-in NIC (preferably) or a USB NIC if you have no available PCI slots. I personally don't care for USB NICs but they can get you out of a bind. I'm not sure how cost effective doing board-level repair is going to be though, since these are really old boards, and a modern, cheap, single processor board and chip is going to run circles around a pair of old-school 533FSB skt 604 xeons.
It's imperative that we repair these mother boards. The mother boards are used in medical applications and the mother board components cannot be modified. Can you repair these boards or do you know someone who can repair theses boards.