any average pc can do MIDI, its when you want to do Audio and MIDI recording that you will need at least a 800mhz pc or higher and some serious equiptment.
for basic MIDI playback you can use a creative soundblaster card which has a basic General MIDI soundset.
for serious use, you will need a MIDI keyboard (Korg, Yamaha, Roland, etc) and a connection method (serial port, gameport, or USB to MIDI) to record into the PC and also to record the audio.
more info available at the Music Tech and ProAudio forums. just search the threads or ask KitC for more questions