At www.musanim.com you can download a freeware program called the Music Animation Machine. If you have a MIDI of your music, you can load it in and you'll have a nice visual experience awaiting you. I had heard about it months ago... it looks like it's associated with the "Tapper" program that was mentioned before. Anyway, I somehow came across it again and have been animating some of my works with it, which you can see here:
I used a free screen capture program called CamStudio ( http://sourceforge.net/projects/camstudio/ ) to record the program as it played through a MIDI. I imported the resulting AVI into Windows Movie Maker, which should preinstalled on Windows XP. I added some titles and credits, then I imported my original MP3 rendered with GPO. It takes a bit of trial and error to get them aligned and synced appropriately, but afterwards, viola (er... voila), a wonderful visual experience. I exported a wmv file, uploaded it to YouTube, and became rich and famous...
Seriously, though, the program is a lot of fun to play around with!
It seems to work best with pieces requiring a small amount of instruments, like a trio or a quintet, otherwise it can get bogged down... like my "Dragon of the Mist" did...