Nothing/Anything.
Before you buy an add-on
serial port card, make sure you really need it. Embarrassingly often,
we've popped the lid on a system, intending to
install a serial port, only to find that there was already a
perfectly good one unused. In our defense, those little DB9
connectors are easy to miss in the tangle of cables on the back
panel, and if the port is disabled in the BIOS, Windows
doesn't report it as present.
So, first make sure you don't already have what you
need. If it turns out that you do need to install one, think again.
Serial ports consume a lot of resources for the functionality they
provide. You may be better off installing an inexpensive A-B serial
switch and sharing one port between two devices—for example, an
external modem and a Palm cradle.
If you absolutely need to add a serial port, buy one of the
inexpensive (~$20) serial port cards commonly sold by computer stores
and mass marketers. We've used many different ones,
all of which seem to work fine and none of which is noticeably better
than any of the others.