i need to make room for an additional card and am planning on a new externall modem.  i have access to an external serial port modem.  how will it compare to a USB model?

   I have never used a serial port modem.  The serial port is an old convention.  Surely USB2 would be much faster than the serial port, but I don't know.

Here's a quote regarding the serial port:

The physical serial port on the back of a PC, must pass data between the computer and an external cable. Thus it has two interfaces: the serial-port-to cable and the serial-port-to-computer-bus. Both of these interfaces are slow.

If you want to read more link here:

http://tldp.org/HOWTO/Serial-HOWTO-5.html#ss5.1

The title of this section is "Is the serial port obsolete?"
I think that would indicate all you need to know