The current firmware for the 396 is 3.60, available here:
http://www.garmin.com/support/collection.jsp?product=010-00425-00
The release notes between 3.20 and 3.60 are as follows:
Changes made from version 3.50 to 3.60:
* Corrected display of communication frequency displayed on Airport and Nearest pages for airports that have multiple Unicom or Multicom frequencies.
* Corrected issue that could cause the odometer field value to become inaccurate.
* Corrected occasional unit shutdown during powerup.
* Improved Aviation In communication format by preventing duplicate waypoints from being added to the route when the remote unit is navigating a procedure turn.
* Improved address search when using postal codes.
Changes made from version 3.40 to 3.50:
* Corrected issue that could cause flights to be recorded while driving on land when the unit is in aviation mode.
* Improved alerting for airspace with upper or lower bounds specified as AGL.
Changes made from version 3.30 to 3.40:
* Corrected selection of duplicate aviation identifiers.
Changes made from version 3.20 to 3.30:
* Improved display of TFR times between 12:00 PM and 12:59 PM.
* Improved airport tower frequency display processing for airports with more than one tower frequency available.
Nothing really major I'd say, but if I were you I would always run the latest code on your unit. That's what I do with my 296.
Ciao, Luca