Just received 12 new 755's from Dell and immediately had issue with onboard NIC. We kickstart our workstations via an HTTP server.
When boot from cd it complains that it cannot find any NIC. What I had to do to resolve this issue is install another NIC (Intel PRO/1000 GT) and kickstart the workstation from that NIC.
Once system is loaded, removed external card, removed all cards listed in "neat"/"system-config-network-gui", removed all alias's from /etc/modprobe.conf file, then rebooted. When kudzu comes up checking for new hardware, it will say that the external NIC was removed, remove the configuration.
Once systems is completely up, open up network panel and add new device, should be onboard chip listed as "eth0", if not, rename it to eth0.