The good news is that you can get a GPS that lets you upload maps. However, in the spirit of proprietary devices everywhere, you need to use Garmin software to load maps onto Garmin GPSes (other software can upload routes and waypoints, but not maps). The shareware GPSMapper program will let you create your own maps and upload them but it doesn't seem to know how to read the commercial maps that I already have in electronic form, so it's not really that helpful unless I want to trace my maps myself. So, even though I have TopoUSA, which has map coverage of the Western US, I need to pay Garmin $100—and that only covers the National Parks in the West of the US. If I want to hike East of the Mississippi, I need another CDROM. And if I want trail maps at high resolution outside the national parks, I'm SOL.
The next time you hear someone talking about Open Standards, what they're really saying is that you shouldn't have to put up with this kind of thing.
My experience is that a GPS can tell you where you are and can tell where you have been, but it cannot tell you where to go (without a lot of doubling back on way points). If you come to a point in the woods where 3 paths converge and you want to know which to take, a map is your best friend and a compass its greatest companion.
Yes, a GPS is a valuable tool if you are in a tank and want to put a round down range with accuracy. Or if you are driving on the Interstate and want to know where to turn for the nearest Pizza Hut. But for hiking, I'm convinced they are more trouble than they are worth.
Well, I absolutely carry topo maps and a compass, but the nice thing about the high-end GPS is that you can download a trail map, so you don't need to haul out your paper map most of the time. Also, it has an electronic compass. What I want a GPS for is primarily as a double-check to tell me if I'm actually on the right trail. That way you don't have to keep asking yourself if you took the wrong turn at that poorly marked intersection back there.
So, there's a bunch of documentation on how to do this at:
http://gwprojects.orcon.net.nz/gps/gps.htm
and to format-convert the data you have, you can probably use OGDI:
http://ogdi.sourceforge.net/
(Note: you're lucky you've got a Garmin. Magellan GPS just can't do this at all.)