Any micro SD card will work. I've got a 16gb in mine and there is a LOT of storage there. I keep all my books on the card. Got about 500+ and it's using something like 2% of the storage.
I use Calibre to maintain the collection. Books are put on the card in directories (folders) by author. If you view by folder view (or whatever nook calls it), you just go to the author and open that, and then select a book. Pretty easy.
I think one could modify this by creating genre folders (mystery, romance, etc) and move authors into these.
Forget about using the nook shelf system to do this.
You can NOT delete books you sideloaded from the nook. Only from the computer.
Hope this helps.