First of all, make absolutely sure that you get a ROUTER, and not a switch (contrary to what some people here suggest)! Explanation: your cable modem only assigns one ip address, that is, it works with only one device directly. A router is a device which acts as a gateway between your local network (your iMacs) and your modem. Here, the router actually creates the local network by assigning ip addresses to the iMacs and "routing" both of them through the single connection provided by the modem.
Even if you don't need a wireless, you will probably have to buy one, as modern routers usually come with wireless. Still, that shouldn't be a problem, as you can easily turn the wireless off. And who know, maybe one day you'll get a smartphone or something, so that wireless could come in handy.