Try to boot using a live distro. If you get the same boot messages with this device, you may have a hardware problem. If this is a card, you could powerdown, unplug the network cable and reseat the card.
I had the NIC interface seemingly go bad on my HP desktop. I even rebooted into Windows, and Windows also said the device was bad. However, after powering down for 30 seconds, the device started working. I think this was caused by a voltage spike on the network cable (router needed rebooting).
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Using Google, maybe the device isn't supported in RHEL 5. This CentOS link indicates that it is in 5.2, but it does provide instructions for earlier versions. Sometimes you need to install a package for firmware for certain devices.
http://www.centos.org/modules/newbb/...topic_id=13720
Also since you have purchased RHEL, if you can't fix it yourself, you can make a call to Red Hat for service.