I have tried to do something similar with little success so far, but I have always been short of time. I do have a few suggestions that you can experiment with however.
First of all, here is what I have found for NON-live visualization. It's quite easy to
play music off of iTunes or Windows Media Player, put the
visualizer to full-screen, and hook up your laptop to a
projector. The problem I found with iTunes is that, at least on a macbook, when you make the itunes
visualizer full-screen for the
projector, even after turning mirrored screens off (ie. the
projector and your
screen can show different things, like having a second
monitor), it works fine on the
projector, but your own
screen goes black. You cannot view your itunes to choose songs.
For visualization of live music, I would recommend finding software that plays back the live input from your computer (
microphone jack), and hook up a feed from the sound board's
monitor output, record out, or headphone output into the computer (you'll probably require a cheap
adapter cable or
plug from radioshack).
The next step would be to get a
visualizer for this program if it doesn't already have one. This is where I ran into problems. It is relatively easy to download free visualizers for audio programs, but I haven't found software that supports live audio monitoring AND free visualizers. Of course, I only have a
mac, and I am therefore limited to a small number of programs and "free stuff".
For my next show, I will look into the same setup on a Windows, and I recommend you do the same. Just search around and you will probably be able to find something eventually. Using this same setup, you may even be able to record the show, unless you already have separate hardware for this.