Sheesh! I had to do some hunting, but I knew the answer was in these forums someplace...

http://www.thesupplynet.com/

Mainiac1957 recommended this site. It's the cable he used and I got one as well. Seems to work great - connects the GPS and PDA to one power cord, and you can transfer data between the devices. I hang the PDA from my rear-view mirror and have the GPS attached to the window and have an up to date map in front of me all the time.