hey man, that's what im in the process of doing on my upgrade from a 12dx to a 20L.
i dont think you can go wrong with a 10gallong sump for a nano. adds enough water voulme to make up for the difference in rock displacement.
i went with a standard 4 baffle setup up: one hanging upside down (in), then one sitting right side up, another right side up (to form the dsb) and another to divide the return area.
very smooth flow. ill get some pics up pretty soon to show you how it works.