I generally use 2-4Meg cards, depending on how much shooting I'm going to do. Most pros recommend using more cards with smaller memory, rather than trying to stack everything on one large disc.
That way if you have a card failure, (which does happen), or something else should happen to one of them, you don't loose your entire trip. I also keep all my memory cards in a "film safe" bag when I travel, just for extra protection.
I actually don't worry too much about speed, but if I they are available when I'm buying, I try and buy the fastest ones I can get.
4Meg cards work well for me on dive trips. Shooting RAW I get 173 shots per card.
I use one card per day. On dive days I use 4Meg cards, usually getting a 2 tank dive trip and any addtional photos I may take throughout the rest of the day. Switching out the card the next morning, during set-up, and lableing the one I take out with either "day 1, 2, 3..." or the actual date.