Not enough main memory speed!
Here I am replying to my own message. To wit:
NOT ENOUGH MAIN MEMORY SPEED!
I figured out what was going on. My lovely GEForce motherboard had mysteriously turned my memory speed down to 200MHz. That should still be more than enough to run 1680 x 1050 x 32, but apparently the NVidia software didn't think so. When I explicitly put the memory back to 400MHz (DDR) and rebooted, the NVidia driver presented the 32-bit option without any fuss.
(I'm still puzzled why 1600 x 1200 x 32 was being offered at the same time that 1680 x 1050 x 32 was not being offered, but far be it from me to question success.)
By the way, if you download the Acer monitor driver from the Acer website (it's NOT shipped with the monitor!!) then you don't need to go into Manual mode with the NVidia software. And you'll get a color profile installed too.