I'm going to assume that you don't want to buy sound-generating hardware to replace your Mac.
Apple provides a free lightweight application called "AU Lab". It is not installed by default. Among other places you can get it at: http://www.apple.com/itunes/mastered-for-itunes/
AU Lab allows you to connect MIDI channels to sound modules called "Audio Units". Audio Units are the standard for instrument and effect plugins in the OSX world.
So you will also need a piano audio unit. It's possible that you'll find the standard GarageBand piano is an audio unit which you can access from AU Lab, but if not you'll need to find a piano audio unit you like. A Google search for "audio unit piano" reveals several, ranging from free to very expensive.
Of course, you're not limited to piano sounds, and some there will be AUs that are less memory and CPU intensive than a realistic piano simulation.
Note that any AUs you install can also be used from GarageBand, Logic and most other DAWs.
Find an Audio Unit plugin that makes the sound you want. Install that. Install AU Lab. Configure AU Lab to route MIDI from your keyboard to your AU. Leave it running.
- KVR Audio has a searchable database of audio plugins.
- 4Front Piano is the first free piano AU I found on KVR. I have not tried it myself, so this is not a recommendation. It claims to be "small on memory and CPU usage".
However this is a reasonable amount of trouble to go to, when it seems to me that simply leaving GarageBand running would do the job. Leave it open on a song with only one track, a piano track.
GarageBand takes a while to load, but if it's not recording, playing back, or monitoring a real instrument, then its CPU load should be minimal, and it should not be doing disk IO. Although its memory footprint is large, most of it will be swapped out to disk, leaving only the blocks necessary to handle piano playing in RAM. Indeed, if you work your Mac hard with other memory-intensive tasks, then try playing piano, you'll notice a delay while it brings those blocks back from swap.