you can use a card or you can use a USB adapter.
i typically go with an adapter, because then you can be sure that anything given off by the tower itself is not affecting the wireless connection.
D-link hardware sucks because they suck at just about everything but when buying a wireless adapter thats what i usually go with because they're the most cost effective, and wireless adapters do seem to have a given shelf life.
i've purchased expensive "high quality" units in the past and they lasted just as long as the Dlink units and were no faster.