What os are you using DL.
If it is Linux your using(i never know with you
) and both your nic`s are showing up ok(ifconfig or lspci to check) then it`s merely a case of going into the network settings and setting one to DHCP for your internet/modem and giving the other a static ip(best way i think...192.168.0.1) with the appropriate subnet mask(in Ubuntu that gets set automatically in the network manager when you click on the empty field)....leave the gateway empty.
Then, install firestarter,go through the wizard setting the appropriate nic(eth0/eth1 etc) for your internet & the other for Internet Connection Sharing on the next page.You can use the DHCP server in firestarter for multiple machines but i`ve always found giving your connected machines static ip`s works better.
Click "save" once your done and if everything is connected properly then firestarter should run ok.If theres no problems just close firestarter once done.
Adding another pc etc is as simple as giving it a static ip(192.168.0.2) with the appropriate subnetmask and the correct gateway...i.e the second nic`s ip address.I imagine it would be much the same for an Xbox although dont quote me on that.
If you turn round and say....but it`s Windows......
The firestarter guide i posted is easy to follow btw
EDIT:AS zing said you do need a straight ethernet cable for modem to pc and a crossover cable from the second nic to >other device<
EDIT2:You can do all this via iptables yourself if your brave enough of course.