First of al, I suggest you get a better NIC (like the D-Link 530TX+) because your current card is very buggy and incompatible with different switches and hubs (search on google "Cabletron 21143 + linux" without the quotation marks).
Apparantly, RH should have auto-detected though, it's pretty known to work under linux (even if with bugs, anyway).
Try using Red Hat's hardware detection wizard to configure your nic..