Hi Tom,
Welcome to the forums.
Let's break it down in detail:
1. You need a way to go from MIDI to your computer (most likely USB or Firewire). Most MIDI interfaces are USB unless they also have an audio interface.
2. You need software on your computer to act as a DAW (digital audio workstation).
3. You need a set of software samples or modeled instrument to create the sounds you want. This will likely be in VSTi format for a PC (Mac uses other formats as well), or possibly as a sound font.
4. You need a way to play this back or render it into audio format. This is generally an audio interface or sound card. This is where it gets tricky as most of the latency occurs between the software and the audio interface. Having a good audio interface increases the quality of the sound and minimizes the latency, you want to look for a device that supports ASIO drivers.
Now as to which platform you are using:
Mac already comes with everything you should need except for the MIDI interface.
A PC laptop will generally not have a great soundcard. ASIO4ALL is a generic ASIO driver that can sometimes improve the latency.
You also want to have your samples on a fast hard drive.
For PC tips, check out the site
http://www.pianoclues.com for some tips on how to set this up and record both digital an acoustic pianos. Someone else would have to chime in with tips for Mac, as I have not worked with Macs.
I use a Firebox for both MIDI and audio interfaces.
For a DAW,, there are plenty of programs out there. I use Reaper from
http://www.reaper.fm .
For software pianos, there are a ton of them. I just picked up Alicia's Keys from Native Instruments
http://www.native-instruments.com . It's currently my favorite and it's only about $100.
One program to make sure you download is MIDI-OX. This program will let you view the MIDI messages so you can debug your keyboard and make sure you have everything connected right.
Rich