I can also confirm 16 GB SDHC in my 765T works great. As mentioned before, HC means High Capacity. The speed is given with the Class number. Usually the SDHC have the classnumbers 2, 4, 6 or 8. Maybe faster now, I don't know.
I bought a Garmin 765t because I wanted BT stereo for the motorcycle. There are many features IGO has that garmin does not. Can you Modify screens like in IGO? Could I load Zumo 660 firmware to get additional features? I have searched several forums and don't see many Mods available. Like different color schemes or modifying the selectable fields.