First off, on the forum, I spotted a very brief reference to Ibycus maps. Internet searching lead me to comments from posters that IbycusTopo21.zip was a great new topo map, and the author had been working on a USA topo map. I spent more than an hour downloading it, and installing it, finding the massive 3Gig ZIP file created a larger topo map for Canada. Oops. Canada wasn't what I wanted.
I then used Garmin's MAPSOURCE.EXE and the already installed California Topo files (in the same C:\Garmin subdirectory) to re-select SF Bay Area sectors, and I this time installed it NOT DIRECTLY ON THE NUVI 750 but instead on my Sandisk 4Gig HDCD mini-disk, which was already inserted in the Nuvi (which happened to by drive J: on my computer), using my USB cable connection. This time, when I fired up my GPS, I selected ALL the maps, and the Topo lines appeared right along with my normal Street names and other features.