I’m often asked how big and how many memory cards that I carry in my camera bag. As may (or may not) know, I tend to be a bit of a minimalist when it comes to my photography gear. That aspect usually dictates what I buy and what I carry with me. If I don’t need it, why buy or carry it.
Right now, I only use two memory cards with my D90. I carry two 8GB Delkin Devices eFilm Pro cards. They can store 541 RAW images each, which is more than I will usually need for a shoot. If I was shooting weddings all the time I would probably have four of those cards instead of just two. (Usually a bride expects at least 1000 or more images.) Two cards would get you there, but you need a spare and you can never shoot enough at a wedding. But I don’t want this to turn into a post about shooting weddings.
I know a few photogs that want to shoot with smaller cards because they want to fill them quickly and then change. They are paranoid about the cards going bad and so they don’t want all their eggs in one basket, so to speak. Changing cards frequently in the middle of shooting annoys me to no end so I prefer larger cards with greater capacity. I don’t worry about losing images because I almost always have my laptop with me to load my images onto and I ALWAYS format my cards in my camera before I start shooting with that card again. This can save you a LOT of problems and it only takes a few seconds to do.
The best advice I can give you is to use a memory card with enough storage capacity to cover the amount of images you need for your given subject. I know that’s a pretty vague answer to the question, but it really comes down to personal preference.