Most PCs sold in the last couple of years already have a "Sound
Blaster" card equivalent built in. It's what plays any files with a
.MID extension (such as you can get from my Web page -- bet my ragtime
compositions would sound great on that Roland digital piano!) Try
them at http://hometown.aol.com/knudsenmj/myhomepage/index.html.
If somehow the PC does not already have a MIDI function with external
MIDI port, then Sound Blaster or Turtle Beach are both good. Make sure
it covers waveform synthesis; the older FM synth method makes for
terrible piano sounds.
So the only hardware you need is the MIDI adapter cable that Robbie
mentioned. It has a DB9 9-pin connector at one end that goes to the
PC's joystick port (games or keyboard, one at a time, take your pick
:-) and the round 5-pin MIDI connector at the other. You are much more
likely to find this cable at a music shop than any computer store.
The same goes for the software. Robbie's recommendations are right
on: fancy notation is not needed, and adds greatly to the cost. One
I would avoid is Midisoft Studio.