If you want to save yourself some money, and your imac has the airport card, you can use almost any wireless adsl modem router (make sure it has the modem built in if you do not have a DSL modem.)
You do not need an airport base-station, which, IMHO, while they look nice, are expensive for what they are.
I am using a totally uncommon wireless adsl modem router with 4 ethernet ports and it works flawlessly with the Macs - I am also using AOL as my ISP (have been with them 7 years) and also have xbox live for 360 connected to it too, no problems whatsoever.
A lot of people use the D-Link adsl modem routers, and the Netgear ones for example, the one I have is an eTec - which is very uncommon.