Buy a router. It's cheaper than another modem. You can buy wireless card (or use USB) for the PCs so you don't have to worry about running wires through the house. They are extremely easy to set up and configure. With my cable modem all you had to do was basically plug the wires in, there was no configuring on the router itself. With DSL I had to enter the account name and password.
Go to
http://www.techbargains.com/ to look for a wireless router. You should be able to find a good deal there. They show a Belkin for $35 right now. I've seen deals for as cheap as $4 after rebates.