1st you should check out this:
http://tldp.org/HOWTO/IP-Alias/index.html
Shows you how to assign 2 ips to 1 nic. I havent ever tried it but I dont think its very hard.
There are several howto's on
http://tldp.org that deal with Masquerading which is what you need to route your internet connection to the other machines.