All data transmissions, digital or analog, are only as strong as their weakest link.

honestly, just get a wireless router, put it in a central location of your home if you want them in different rooms or in the room next to the one you are using to house the most computers (NOT THE SAME ROOM!) use a wireless G router (the higher data rate allows you to go further away from the router while still maintaining good connection speed) and you are basically set since you can put your modem anywhere you really want as long as there is power and a cable jack.

BUT some cable companies use routers and switches to get you your stuff, and the more computer you have on the router the more you are demanding internet, the more the cable company give you for the time being. all this being an unintentional flaw in the "lets distribute power to where it's needed most" system. IE mrs. grandma isnt hogging up everyones internet when she just checks email once a day.