Actually - I ran out of time on my last post, but there is a new system called WAAS "Wide Area Augmentation System" that in addition to the sat signals, adds ground stations to give the same level or better than, the accuracy of DGPS - cheaper too. I was looking a couple weeks back and found this stuff on Garmin's site - here's a link...
http://www.garmin.com/aboutGPS/waas.html