pretty much any card that is a reputable brand. The 600SC is a pretty solid system, I would be surprised if the nic actually failed.

I need something that will fit in the 64 bit slots as I have already used up the one PCI slot. I plugged the network cable into a laptop and it worked fine, so its not the switch or the cable.

Any 64-bit or 32-bit PCI/PCI-X card will probably work.

NOTE: A 64-bit PCI-X slot can easily handle a 32-bit card.