Ron, about the texture sizes not fitting in memory, would it be possible to have the textures saved to disc and load only the textures visible and those next to them into memory? If zoomed out then 256x256 versions of the textures might suffice. Unless you're madly spinning the globe around (at which stage we could have just 128x128 textures for the rest of the map quads preloaded to use) then you should have time to load the textures up into memory and discard the ones that aren't needed anymore.
This would essentially be like google earth but on a simpler scale and direct from your harddrive not over the net.
" There's nothing like a bit of destiny to get the ball rolling " - Terry Pratchett - The Wyrd Sisters