If you search Net54 for "silica" you will find a number of threads about this:
http://net54baseball.com/search.php?searchid=1428414
The desiccant packs you buy at the store are mostly filler - the active parts are the small blue/pink crystals. I used to work in the rain forest and used only the silica crystals (aka gel) and no filler and it worked well. At the end of each day I put electronics in a sealed dry bag with silica in a sock.
When they are pink they are ready to absorb water. When they are blue they are "full" and need to be baked in the oven to recharge/turn pink again. I was told to use a dedicated pan for this as the silica isn't something you want to ingest. For cards you might put the silica in a small tupperware with holes you poked in the lid just to make sure there is a physical barrier between it and your cards.
With all that said, I don't use any desiccants with my stuff.
