Thanks for your reply, Boyd. I appreciate your help and knowledge (I've seen you in other threads I read prior to creating this one).
As far as the internal memory goes, I didn't mention it because I want to keep all of my maps on my SD card. But I did test it and found I need to have the files named very specific names: gmapbmap.img (default world basemap, didn't overwrite it but I assume it would work), gmapprom.img, and gmapsupp.img. I had tried another one or two that didn't, like gmap3d.img and I forget what else. As mentioned before, the only name that seems to work from the SD card is gmapsupp.img.
I found the section you quoted from the GarminDevice.xml file, and it is definitely in there. So now I'm even more confused!
I had also tried editing that file (backed it up, of course) and specifying different BaseNames to see if I could force it to read a different name (or wildcard) instead of, for example, gmapsupp, but that didn't work either. I guess that file is a log of some sort rather than a configuration file.
I actually don't have access to a Windows or Mac machine right now. I work on Linux machines for work, and my local family either has no PC or asked me to convert them to Linux already.. I am fairly confident that the files aren't being renamed something wonky somehow because I believe I am pretty tech savvy, and because I can get the maps to load when I manually rename them to gmapsupp.img.
I had actually contacted Garmin support via email before I posted here, but I don't expect a reply as quickly as I could get from people like you.

Besides, I didn't know if they would want to actually support me, using free alternatives to their official (and pricy) maps.
EDIT: I just thought of a couple other things to mention. When I first started out, I began with a brand new 4GB SDHC Class 4 card, put my maps on it, as well as a Geocaching.com Pocket Query GPX file. The maps weren't showing up, so I tried on an older 2GB SD card I had kicking around, and they still didn't show up... I thought there was something wrong, but then I noticed that the Geocaching stuff was being read with no problems. So I don't think it's the SD card itself, which was my first suspicion. Also, the maps I've been testing are not huge. Maybe 200MB each, max, so I don't believe there is an issue with sizes or something, especially since it doesn't work if I put just one single IMG file on there with a name other than gmapsupp.img.