Google {"USB modem" +56K} (after removing the braces). Apparently US Robotics
sells one with a Linux driver. By going to an all-in-one USB Modem you end up
with a smaller kit and you know for sure that the modem for sure will operate
with your USB port. If you buy a USB-to-serial adapter and it doesn't work, you
probably can't expect much support from either your modem manufacturer or your
adapter manufacturer.