Q: My 3com card doesn't work!
A: Okay, first, look at this bugzilla bug. There are known problems with the 3Com 3c905B/TX and how it interacts with kudzu.
If you want to disable kudzu (which detects when you have new hardware) this will resolve the issue for now. As root, in a Terminal, type chkconfig kudzu off -- after you reboot, your card should work. (Thanks to jroysdon for pointing this out.)
I personally have worked around these problems with the following process. Note that this process is probably not reccomended by Red Hat, since it uses non-standard drivers. If you use PCMCIA (those laptop cards), this may cause you trouble.
You must have kernel-source installed for this to work. Check the "Add/Remove Applications" tool in System Settings on the red-hat menu.
Download Donald Becker's netdrivers to your home directory.
Open a Terminal
Do cd ~;mkdir netdrivers;cd netdrivers
Unzip the netdrivers using tar -xzf ../netdrivers*.
Compile them using make CC="gcc32"
Do make install.
Restart your computer, but watch out for kudzu. That means that if a blue-and-red screen comes up asking you something about 3Com, say "IGNORE"