To follow up on what Dan said, you could also consider using MP3 files on stage. I know several people who do this currently. Portable CD players that are MP3 compatible are under $100 and pretty common now, and you could possibly get enough MP3 files on one CDR to do an entire gig. Also MP3 players with a hard drive would work such as the Creative Labs Nomad (http://www.nomadworld.com). I use one for practicing.

I disagree that the softsynths sound bad - I currently use Yamaha's latest version 4.0 of SYXG-50 on my Windows 2000 laptop and it sounds pretty good - maybe not as good as my normal keyboard rig, but good enough. Helps if you know how to tweak midi files to get the most out of it, of course.