Maximum RAM in a Mark 2 is 16MB. Mark 2A has a somewhat solid path to 32MB, with potential, with extreme risk to go to 48 or 64MB.

http://www.eutronix.com/media/memory_manual.pdf for the hardware side complete with howto.

Software side, I don't remember offhand how stable upgrades were. Most of the discussion about it happened in 2004, and kits were sold for a short time.

Google may offer a better search then the boards to see what people's experiences were:
https://encrypted.google.com/search?q=empeg+memory+upgrade+site:empegbbs.com