There is nothing specail about the cards for the etrex, any standard microSD will work (micro not mini), no "proper" card. What we were referring to is garmin sells cards with maps pre-loaded so you don't have to have software on a PC to install the maps.

IIRC Wine will work under mac and Mapsource runs under wine, thats how I loaded my maps, just loaded everything then use the USb storage mode to pull the gpx files off after that.