Actually, today's civilian GPS systems are very accurate, down to centimeters given the right circumstances. When GPS was first put in to use, the military had a program called, Selective Availability (SA) which made civilian GPS systems not nearly as accurate when compared to the military. Then, President Clinton ended the program of Selective Availability, giving consumers a much more accurate system. A civilian GPS unit uses one channel to receive satellite data, while the military uses two channels. If you stopped the comparing there, then yes, the military version can get more accurate. However, there are things called, "Augmentation Systems" that civilian GPS units can take advantage of for greater accuracy. One such system is WAAS (Wide Area Augmentation System) which can greatly increase accuracy. Another augmentation system is currently in use by cell phone towers. They are also equipped with GPS monitoring systems that give automatic corrections. These augmentation systems when used on civilian GPS systems can give as good or better than just the standalone military units with their two channels.
Back in the early days of GPS, your statements are absolutely accurate, but much has improved. More information on today's GPS constellations and augmentation systems can be found here: www.gps.gov
As related to the discussion, I think a GPS integrated into a detector is a great idea. Not only can it possibly prevent you from covering ground you've already explored, but you may also have the capability of marking your finds, then downloading them to a map on your computer later. This would give you the ability to notice patterns and give a better idea of some other areas to explore in and around the area that could be promising.