Size of memory card kind of depends on resolution settings you use. If you shoot in RAW camera format, TIF, or Large/Fine JPG these files take more space so you'll be able to fit fewer pictures on a card. There is a trade off of how often you want to swap cards. Most of the cards are very robust but there are times that you may lose a card or have its contents unretrievable. In that case, having all your pictures on one card would be a bad thing. I typically select a size to give me enough pictures that will get me through one event (parade, party, game, etc.) to minimize missing action while changing cards. I don't get the largest cards though just because I am paranoid that if something happens to it I'd lose everything. With my Nikon D100 I shoot 1 GB cards with 80x write speeds in Large/Fine JPG format and rotate between 3 cards. It's worked pretty well for me.
Jeff