The Dell Dimension is a desktop computer and will need a wireless card or adapter to connect to the internet.
look on this page. http://support.dell.com/support/edocs/systems/ins5100/en/sm/index.htm They show you how to open it up and everything.
USB wireless card is convenient just plug and install a driver
(check the related link)
Yes, it's. You have to purchase either a PCI wireless card (you will have to open the computer case to install it) or USB wireless adapter (which is easier to install, just put it one of usb ports).