Falling back on our trusted DLink (realtek based) 10/100 pci card to get us onto the internet, we downloaded e1000e from Intel and  sf.net, tried igb from the latter source..... no joy. BTW you need to install the dev tools and kernel header to compile things on ClearOS5.2 - yum install gcc kernel-devel does the trick.

At this point we decided to take take the plunge and look at the kernel itself, as we came across reports stating that the card was well supported in later kernels. Following the instructions by Tim Burgess here we were able to install a more recent 2.6.32 kernel. We then downloaded the kernel headers for that kernel and installed with rpm -ivh.
Moving into the e1000e driver's src folder we did a make clean & make install followed by a modprobe e1000e and like magic we were presented with an eth1.

Apparently there are some minor issues in using the more recent kernel, such as loss of functionality in the protocol filter section. As we were not using this particular machine as a gateway but rather as a file, email and groupware server, we could afford to sacrifice this for the sake of getting the onboard card going. If you have a need for this functionality, the howto which Tim has written provides some instructions for alleviating the pains....