Though I'm on DSL now, my trusty U.S. Robotics 56k External modem is the best modem I ever had. It was better than any internal modem I ever used, including ones made by U.S. Robotics.
Never had problem with getting kicked off-line, and I always connected at 53K (highest you can connect).
It was $100 when I bought it, but you probably can find it cheaper.