Need a little guidance from those of you with the Garmin 276/etc series. How big of a memory card does it really need? I started playing with it today, and assuming I want to add all Topo for East Coast, Maine to Florida, out to Pennsylvania and down? Or is that like computer memory, buy as much as you can afford? First time trying to buy these larger cards, and they arent readily available at local stores to even see what they look like and read about. Any help appreciated before I make the purchase.