I looked around a bit and figured out that you motherboard uses the 875P chipset. I then found a review for a Intel motherboard using that chipset that has gigabit lan;
here
In it it said that they had to go to the newest pre-release kernel that is aviable to get the LAN to work, which was 2.4.21-pre7. The kernel that comes with redhat is 2.4.20.something.
Right now the current stable kernel is up to 2.4.23 so you may need to update your kernel in order to get the support for the new chipset stuff.
Now the standard way to do this is to download the kernel sources from
www.kernel.org and compile a brand new custom kernel from scratch, but that's a pain to do for a newbie. Your welcome to try it, if you mess up you can still switch back to the old kernel if you want, but it may not be fun.
You can get the newest kernel tar.gz sources from here
There are quite a few online resources on howto deal with compiling a kernel and what to do to get the bootloader to use it. A few searches on google will provide plenty of information. It's worth trying a couple times, IMO.
However if you just want to try out Linux with out all the hassle of learning to compile junk maybe try out one of the new releases from Mandrake. The newest version is 9.2 and uses kernel 2.4.22. Or maybe upgrade to a kernel from Fedora, which is the successor to Redhat's numbered versions.
You board just post-dates Redhat 9.0 that's all.
I am not 100% sure if the newer kernel will fix your problem, It's all based off of that review and educated guesses.