I used the SD card to store additional maps, mp3 files, and audible books. I even pull out the SD card from my digital camera and use the nuvi to look at them because it's screen is much bigger.
I know for a fact, the Nuvi 760 (other models may vary) can use SD and SDHC cards at 1GB, 2GB, 4GB, and the awesome 16GB size.
The Nuvi 760 can use 16GB SDHC Cards. I have used both of these in mine
A-DATA 16GB Secure Digital High-Capacity (SDHC) Flash Card Model TurboSD SDHC 16G
Transcend 16GB Secure Digital High-Capacity (SD/SDHC) Flash Card w/Compact Card Reader S5 Model TS16GSDHC6- S5W
Hope this helps
PS I find my nuvi is a bit faster when the MP3 files are in different folders. Also, the 2GB and 4GB SD cards are a good compromise between tons of music and waiting on the Nuvi to load the directory of music--the 16GB card can take a bit of time to load roughly 2500 MP3 files... I find that the 4GB card is optimal--a different card for a different type of music. It is worth the money to get the high speed SD and SDHC cards (and never anything less than a Class VI card on anything over 2GB).
Here is what I do to charge my NUVI with just about any usb charger (cell phone, usb cable from computer, etc)
Turn on Nuvi 760
Press and hold near upper right corner (by the battery symbol) for several seconds and release
This will put you into the maintenance mode
Now, plug in your usb charger
In the maintenance mode, you can scroll through the screens--one of them has a battery charging status in percent--you can watch it climb a percent at a time.
Normally, I just exit the maintenance mode by choosing exit near the bottom right corner. then the nuvi will reboot and will be usable just like you are in the car cradle while charging and the normal battery symbol can tell you the state of charge.
The maintenance mode is neat--just be sure to scroll through the screens and always use "exit" to leave it. To be safe, never switch it off from this mode.