Standard serial com ports (not USB) run on the 16-bit ISA bus. But a decent mobo w/good serial ports, ISA & DMA buffering, and peer-concurrency won't be overloaded by any reasonable ISA devices, so com port thru-put will be limited by your ISP connection & not your hardware. In fact, on my so-so box, downloading a large file (via ISA modem) & playing a large sound file via an ISA sound card at the same time show practically no effect on CPU usage, and no buffer overruns, modem packet loss, etc., occur in the modem log file.