For EVERYTHING Palm, I've been going to
http://www.palmgear.com the FAQ on the left side of the page should provide you with some basic info on the serial port (older PALM models). For Visor and USB... That's trickier. You may need to go to
http://www.handspring.com and check for their developer resources page for info on how to get your external hardware setup.
Serial connection is the way to go. Google should have dozens of pages on how to connect a serial port to a breadboard for almost any application.
Good luck! If you manage to get the USB to work, my hat to you, but the springboard module (if you can order just one of the HDK units go for it) is the better way to go. It will be interesting to see how your project goes!