Well, strangely, I had problems with the Intel 536ep - could connect, but little/no data through-put...... - recently bought a
Creative External Serial modem
works perfectly under
Fedora Core1
Slack 9.1
Mandrake 9.2
etc
no extra drivers or config required - in kppp for example just put in
the device name of the modem (/dev/ttyS0 for mine)
connection speed
phone number
username
password
etc etc
you should'nt have problems with an external serial modem.