Last night, I updated my nüvi's firmware to 3.2 using nothing but my G4 iMac, 10.3.9, and a USB cable. It's almost too easy to call a "hint." Download the updater from Garmin. It's a .exe self-extracting archive (boo, hiss). Drop it on Stuffit Expander. Mount an SD card, either in your nüvi attached via USB, or in a card reader.
Drag the Garmin folder from the expanded archive to the card. Put the card in the nüvi and boot 'er up. Let the nüvi do its full update process. That's it.
If you're updating from a version of 2.9 or older, be sure to back up your waypoints file first, found in the main nüvi directory. Again, it's drag and drop. No Windows needed.

