[B166ER]

I'm running into a problem with too much of the main RAM being eaten up.
I have several SD cards with scads of room.
what is the most efficient way of maximizing available room?
I wanted to put essential apps in main memory and non-essential on the SD cards, but I keep running out of room. It seems like even if I specify the SD card as the install location, some of the main ram gets used.
Also, what's the difference between main RAM and built-in storage?
Thanks