I have a 320 also, and just downloaded the free topo map that is posted somewhere here on ETHD. I downloaded the whole state of OK to my Garmin. Dont know if it is the best map or not, but it shows all the creeks, roads, and anything else I think I would need. Plus it was free. It overlays right on top of the birdseye view.
Here is the link.
http://www.easttexashogdoggers.com/forum/index.php?topic=7547.0