| | Re: swap sim card from 8310 to 9300
You would be well advised to start doing some real research before getting yourself into something.
The data plans with AT&T (as with many carriers) are a set price for a set amount of data. (Not "air time".) For example, you might get 300MB for $20 and 3GB for $30. If you go over the set data limit, your carrier will typically charge you a fee.
As noted above, when you use data over WiFi, that data typically doesn't "count" against your data limit.
GPS apps and maps are two different things, so be careful not to confuse the two.
Obtaining a GPS signal and the coordinates does not require data. However, map applications do require a data connection.
They are very few options to "install maps on a media card".