I've gone thru the crypt, toad village, the snake temple and now the castle. I've started to accumulate a bunch of keys and crystals that I can't sell. Am I going to need these later on or can I start dropping them since I don't need them anymore. Thanks

I believe you pretty much use the crystals/keys only where you find them. I can't think of an instance where this hasn't been the case. Other objects are used elsewhere, but I think it's fairly safe to get rid of the crystals.