I have the Garmin Vista (25 gig), Rhino 120 (8 gig), and the Rhino 110 (1 gig). I use the MapSource United States Topo. I comes with three CDs (Eastern U.S., Western U.S., Alaska and Hawaii). If I'm not mistaking, it was about $100 at REI.
When I went to Moab this year I mounted my Rhino 120 on the blank button near the cruise control. I was able to download all the maps from Northern New Mexico to Moab, Utah.
It worked out really well.
Peace.
Tom
"Through Great Sacrifice..... Great Rewards Will Be Achieved"