I feel your pain. I'm also a brand-new linux newbie with your same motherboard, same Red Hat distribution, same problem: No NIC. Here's how I got it to work:
1. Clean up previous attempts. I don't know how to do this, so I just reinstalled Red Hat 9. I'm sure there's a clever way to undo previous changes, but I don't know it.
2. download the source rpm. As you correctly note, the existing binary rpms won't work, as they are for 2.4.20-6 or -9, and your kernel is 2.4.20-8. The source rpm is NVIDIA_nforce-1.0-0261.src.rpm and is available at http://www.nvidia.com/object/linux_nforce_1.0-0261
(bottom of the page).
3. Log in as root. I found that becoming root isn't good enough, as important binary directories, like /sbin, weren't in my path otherwise.
4. Put the source rpm on a floppy, and copy it to your linux box. I put it in the /root directory, which is probably bad.
5. In that same directory, build the binary rpm from the source rpm by typing
% rpmbuild --rebuild NVIDIA_nforce-1.0-0261.src.rpm
a few screens of gibberish will scroll. The only thing that matters (assuming no errors) is the line near the end that reads
6. Go to that directory and rpm that file:
% rpm -i NVIDIA_nforce-1.0-0261.i386.rpm
7. Load the module by typing
% modprobe nvnet
You will get warnings about a tainted GPL license, which may annoy your soul but won't bother your computer any.
8. I don't know how to do this part in shell, only in the GUI (Gnome). Run the Network Configuration tool in System Tools again. You should see your nvnet on eth0 on the Hardware tab. On the Device tab, click "New", select "Ethernet connection", click "Forward", select "nvnet (eth0)", click "Forward", and click "Forward" one more time. After a short delay you should have a usable NIC. Click "Activate" to, uh, activate.
9. ping google.com and/or open a web page to check it. Mine works.