First of all (considering this is your first post)... welcome to GPS Power!
Garmin nüvi 3xx devices can see SD cards larger than 2 GB but not any map files larger than 2 GB. To make maximum use of the available storage options, you can use whatever space is available on the internal memory to place a partial map in the folder Garmin (use the PC installable version of the map and use MapInstall to select the map tiles of your choice, making sure the size of the selection remains within the available storage space of the nüvi). Name the file gmapprom.img.
Note: make sure your PC shows file extensions of known file types, otherwise you may end up with a file named gmapprom.img.img which won't work!
Next you can repeat the process with additional map tiles of the same map, but now store them on the SD card in a root folder named Garmin. Name the file gmapsupp.img. See note above. Make sure the total size of these additional map tiles remains within the 2 GB limit. Depending on what navigation map you have chosen, there may still be map tiles that are not present in the selection for the internal memory nor the SD card. Tough, but at least you have maxed out the use of the storage space available to you for this device.