Not sure if you're using a PC or a Mac, but there's a good program for the Mac called GPSy, you can download a demo, check it out at www.gpsy.com
You can use the nauticla BSB maps, USGS maps, tons of different maps, even scan in your own (like the Fish-N-Map company maps) then calibrate them with the software (figure out the datum, pick three points)
I soldered up a serial-to-GPS cable for my Lowrance handheld, (if you can't solder, you can buy cables on the site) I run it on my boat with a Mac laptop; nice big screen, pretty cool, helped me a lot in the fog up in Barkley Sound.

-N.