If you shop around, it's possible to find a 128MB SMC in the $20~30 range, which is what you ought to go for. One 128MB SMC holds roughly:
-2 CDs worth of music in MP3 format
-4~5 CDs worth in Ogg Vorbis
-One full-length movie/90 minutes of lower quality video; 60 minutes of higher-quality video
-A few hundred NES/SMS/GG/GBC ROMs
-Close to 100 smaller SNES/PCE/Genesis ROMs when in zipped format; around 50 larger ROMs (i.e. 32megabit/4megabyte games when zipped)
-Some 175 Atari ST diskettes, at 720k a pop
- ??? amount of homebrew games (the awesome Beats of Rage consumes around 60MB, but strip away the audio and cutscenes and it's only 7MB or so.) GPWar is around 16MB, Gianas' Return and GPFinalWar are only a couple megabytes apiece, but between all the great original titles out there (Xigon One, Numo, Vertical, Fire&Fire, GiriGiriWalker, Tobi Drummer, GPSquares, Electronia, etc.) it all starts to add up.
-The 424-and-counting episodes of 8-Bit Theater might be a tight fit, but it should squeeze in there...
-Commercial games are all in the sub-32MB range, most under 16MB. JoyGP titles are usually smaller; retail-box-only games (ASR, Her Knights, etc.) are larger, I believe.
Now divide all of that by 4, and that's what a 32MB card will hold.