Well i have 3X 8GB Cards...
One will be for Movies and TV etc, i find reducing a 700MB file to 350-400, depending on movie (i.e if a dark film then 400, if a animated etc then 350) TV shows from 350MB to 175-200, and Cartoons etc 175MB down to 75-100MB seem to give me decent enough quality.
a 8GB Card can hold about 16 hours video (620-640X res 900-1000 VBR) which is average Xvid/Divx quality.
So if i recode then videos i can then get 32-35 hours on a card.
As for CPS1, SNES, NES, SMD, SMS/GG, PCE they dont take up much space.
GBA/CPS2/MVS take up the most.
One Card will have ports on such as Doom and its mods, and also Open BOR etc....