I have a midi composition for piano that I would like to apply a soundfont to, and process out to a wav or aiff file. The standard issue quickime sounds are good but not ultimately what I want. Is there a way for me to add additional sounds, like real instrument samples?

I compose in midi, then painstakingly retranscribe into the keyboard using step programming because there is a problem with my keyboard's midi-in port. I'd rather handle it all inside the machine if possible, without even going to the keyboard.

I realize I'm incredibly novice in regard to midi, so please bare with me if this makes no sense.

Very simple : you can use a sf2 soundfont (there are many free on Internet, reproducing generally a Steinway). You place it in your MacOsX library (Users/John/Library/Audio/Sounds/Banks).
Then, in System Preferences, you set Quicktime (Music) to the soundfont you want. It replace the Quicktime Instruments by your soundfont. If you play your midi file with an appropriate player (iTunes, QMidi, Sweet Midi Player), you will have an excellent sound. To capture this sound and directly translate it to aiff or even mp3, you can use AudioHiojack.

You have no idea just how helpful that was. Thank you so much.

For my own piano midi files, I bought the Realistic Piano software, ridiculously cheap soundfonts of marvellous quality (24 euros). It has a Steinway and a Bösendorfer in many formats and high rendering.

See http://www.realisticpiano.com/