Depending on the system, sampling frequency may not need to be high. I have seen .5 second sampling interval work very well for controlling a slow closed loop system. Many systems such as temperature control could be even slower. A big concern is flexibility. Your PID controller needs to be able to be tuned very easily. I would implement a serial interface with Hyperterminal to allow viewing and editing the P, I, and D coefficients, and an EEPROM, etc for storing them.

Cory Spackman