Mod as in 'modification'. Eris ships with an 8GB C2.
As posted above you, be wary of who makes your C6 card; performance varies -- RiData seems to be the brand of choice.
Say your car drinks gas and seems slow. You wouldn't spend $50 on snazzy synthetic motor oil, when your spark plugs are worn out, air filter is dirty, tires are flat and there's a pallet of cinder blocks in the trunk, would you? Same with the phone -- make sure everything's in good working order before considering a modification.