There is cell phone based wireless Internet access, which takes a special "cell-card" for access.
Then you have your "WiFi" based internet connections; where someone (yourself) has a broadband wireless router connected and configured to provide wireless Internet access to any users within range of the transmitter.
Not sure by what you wrote on what kind of Modem the Dell rep is trying to sell you, but if its V.9x based; otherwise known as dial-up, then the modem should be part of the package with no additional cost involved; but if they want to charge you to include this most basic feature, then have them leave it out; and instead throw-in a name brand wifi wireless card; and see if they offer a wireless card that has a removable antenna, that can be moved independently of the computer tower, this will help you get the best signal possible.
But I’m wondering, why worry about a wireless internet connection when you are buying a desktop computer? Your going to have a mess of cables connected to the tower, what difference does one more cable make. And with a "hard-wired" internet connection, you don’t have to worry about wireless signal dropping out, or having any interference, etc. you are always going to get full speed out of the ethernet port that the computer’s connected to.