Several other connection options exist. Residential DSL is often provided with a combined DSL modem and card, that is added to your PC. This card is all you need. These cards are known as PCI DSL modems, and are designed for Intel PCs.
Another connection option is a USB DSL modem. These are newer and more likely to suffer from incompatibilities wth your PC or operating system, but they have the advantage of working on Macintosh, and possibly USB capable laptops as well.
For small business, you are likely to already have network cards, as you would almost certainly have an existing local area network.
Also read About DSL for lots more information