slabbandit
A 4 or 8 GB SD card is probably all you will need, I also purchased the smallest card I could find, this I use only for making updates to my units.
As far as a navigation cards, Navionics covers everything across the the U.S., Lake Master is expanding so depending on where you live, will determine if you can use a LakeMaster card.
I have both cards and they both have some unique issues both pro and con. You will need to look at what both cards have for the areas you fish. One may have High Definition lakes that you use while the other may not, so be careful before you make your purchase.
George