As others have said... IPMasq will work, but paying for real IPs rather defeats its purpose.

If you have real IPs then you simply need to make a machine that will share its connection with the other machines. You will still need two NIC's, each with and IP address that is provided by your ISP. One NIC will be internal the other external. Test your Linux machine with just the external NIC enabled and make sure your internet connection is working properly. All of the IP info for this NIC should be provided by your ISP(netmask, IP, gateway, DNS, etc.) Then enable the internal NIC and make sure that you can communicate from that machine with both the internal and external networks. Once again, all of the IP info for this NIC should be provided by your ISP. Now you just need to make sure that IPforwarding is enabled as stated above.

Each "client" will have its gateway set to the internal NIC address of the gateway machine. Their netmask and DNS info, should be whatever your ISP has provided. Your machine should forward internal IP info straight through to the external network and then your ISP out to the internet.