I might be a good person to answer this, as I've set something like this up twice.
My last experiance with RedHat was with version 5.2, and that's been many a' year ago, but I'll see what I can do.
As root and replace $EXTIF with your outgoing ethernet, and $INTIF with your internal ethernet,
modprobe iptables_nat
modprobe ip_nat_ftp
modprobe ip_nat_irc
iptables -A FORWARD -i $EXTIF -o $INTIF -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT
iptables -A FORWARD -j LOG
iptables -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
There should be three modprobe statemenst and four iptables statements. After you do that then try to ping a website or something from your Amiga. If it works, then it shall until you reboot the Linux box.
I'm unfamilier with RedHat's startup scripts, but I think that their located in rc.x, where X is a number between 0 and 6. The directory that you want is probibly rc.3 or rc.4, find in one of those a file called network or firewall or something like that, and type the above commands into it, and it should load them on startup.
This is far from an elegant solution. And let me say that I'm far from familiar with Redhat, so please excuse if any information is wrong. Please make backups of the configuration scripts before editing them.
If I may also say, I don't think what you've setup is the best solution. If I were you I'd find an old PC (486's do nicely, in fact my gateway is a 486/33 with 16 mb of ram, and an 82 meg Harddrive, and it works great), put Linux on it (my Preference is Slackware, but and older Redhat or the New Debian should do nicely), and have it be a dedicated router running almost all the time. That way you wouldn't have to wait for your PC to boot up to use the internet on your Miggy :-D
I hope this helps,
Clark