First check: do you know if your can you get your' eth card & modem to work as seperate entities? if not, start there.
I haven't played around w/ this routing stuff much yet, but here's some recources that i usually use for info:
check out the how-to's @ www.linux.org,
or here: http://hints.linuxfromscratch.org/hi...squerading.txt
One of the main differences w/ linux is that a lot of the configuration stuff is done via text files. this does take a while to get used to, but once you get the hang of it, it's not so bad (it's actually nicer b/c you know what is going on i.e. what the programs are looking at for the configuration).
Do you know how to deal with configuring the kernel (or at least looking at your' current config)? You probably will want to research that if you are planning on doing anything like what you are talking about.
I'm a newbie too, and definitly don't know much about how the redhat system works, but i hope i gave you at least a little insight on where to start.
Good Luck (and don't give up... once it's done, you'll have a much better understanding of how a much better operating system runs.)