I'm just running a regular SanDisk 2gb SD card and it fits the western half of the US topo maps fine. I bet I could add some more, but I don't usually go east much, and it's just more maps for the GPS to think about when starting up.
As far as maps, I'm pretty sure only the garmin maps work, but I haven't tried others. I know garmin topo 2008 works as well as the older garmin topo USA (the free unlock one). I also have the 24k national park maps, but haven't attempted to put those on it.
I got the GPS a week ago, and thanks to Garmin's 60 day map update guarantee, I was able to update the preloaded 2009 streetmaps to 2010.