I don't know for sure type of embedded NIC you have(do you have any model #'s or anything?), but on Dell's website it mentioned Intel Gb 82541.
According to this site, the e1000 kernel model might work for the 82541.
I have a SC1420 and it has some sort of Intel Gb on-board NIC, and it appears that e1000 is the module that its using. Its running RHEL4(well, CentOS technically
), and it installed fine. If its an option maybe you could try a later version of RHEL, though from your description it sounds like you have a RHEL 3 install thing all customized for your servers so using RHEL 4 may not be possible.
Additionally, here is the RHEL 3 documentation about making driver disks.
I've never needed or made any driver disks for Linux so I can't really tell you anything more than that link to the RH doc's says. Maybe someone else will be more familiar with it.
If none of that works, you might see if you have a PCI NIC laying around that you could use to install the system and then work with getting drivers for the on-board Intel NIC once its all installed(or not if the PCI NIC works fine and you don't need it for another box).