The modem is your connection to the outside world. Always purchase the fastest available when you acquire the computer. Today, Fall 1999, this will be a 56k modem. Look for the 56k and V.90 certifications on the modem box or in its instruction booklet. Beyond that the only decision is internal or external.
Internal modems are cards that are mounted inside the computer case.
The only cable these require is a connection to your phone line. External modems sit outside the computer case, have their own case, and require both a telephone connection and a cable to attach to one of the computer's serial ports. External modems can be moved to other PCs easily, and their status lights can help with some troubleshooting. Internal modems can be moved, but you have to disassemble at least a part of the computer. Both types should come with a suite of software which includes facsimile software.