Just as a note, Pokemon Diamond/Pearl uses a 128 MB card. However, for some reason it only uses under 70 MB of that space. I can't really think of too many reasons to have done that, but perhaps since higher-storage cards have slower data transfer rates, the Pokemon team was just setting a standard for their games with the load speeds of Diamond/Pearl so nobody would note any significant changes even if they made a game using the full 128 MB (which would be quite a large game).
To compare, Final Fantasy IV on the DS, with all its 3D graphics and voice acting, was a 128 MB game.
The Pokemon team will have no trouble remaking Gold/Silver. In fact, I wonder why they haven't made a full 3D Pokemon on the DS yet since it can probably handle one (though it most likely has to do with the Pokemon team not being too familiar with 3D graphics).