I understand this is a Fedora list. And building your firewall "manually" is a great way to learn firewalling on Linux. However, if you find yourself totally flustered and are about to give up, there is always Smoothwall, wwww.smoothwall.org, which is a turnkey solution you can use.
Having a true Linux box would be preferable, as you can then configure other serverices such as a cache DNS and speed up your lookups a bit, etc...
*> >> Now I have two networking cards, one buildin in the motherboard
*> >> (eth0) and one in a PCI slot (eth1). When I tried to do the same to
*> >> give my WinXP box access to the internet I couldn't get it right.
*> >> When I connect to internet using eth0 everything is fine. When I
*> >> start eth1 to the WinXP box it works, but then I have no contact with
*> >> the internet thru eth0.
*> >> To get contact with the internet again I have to stop eth1 and
*> >> restart eth0. Does anyone have a clue?
