Following up to my own post, a few weeks later, now that I better understand stuff. Essentially, I was thinking in PC terms, where RAM (memory for running applications) is different than storage capacity (typically, hard drive storage).
For the GPS, "RAM" really is equivalent to "storage". A Garmin Oregon 200 w/24M vs a 400 w/1G really means that one starts with a 24M HDD, the other with a 1GB HDD. Both have, I assume, the same amount of RAM for executing the OS.
Now that I've played around, I find that I can load (using MapSource) three different California maps (a routable OSM map, a topo map, and <name escapes me - ???cus maps>) in about 1.2GB, leaving me more than enough room for one-off maps that I load separately when traveling out of the area. 1000 geocaches in a GPX file is WAAAAY <24MB - so if I wanted to add POIs I'm sure I'd have no problem.
What I'm saying: my experience is that, for a single state (California), I'm hard-pressed to use more the 2GB of storage, so I ended up making quite a fortuitous "mistake" when I ordered the Oregon 200 instead of the 400 as I had intended. The lack of audio bell and electronic compass have turned out to be non-issues.