Also, please remember that capacity depends on what quality level you are shooting ... RAW, TIFF, both eat up a lot of memory (but worth it) where as shooting the various JPEG quality levels will allow many images to be stored. Kenneth's question is absolutely appropriate. You might want a larger capacity card for a prolonged day in the field, but as soon as the shoot is over download and erase.
PhotoJack also brings up some good points ... I "invested" in a 16GB for my D700, but am sorry I didn't buy several smaller cards which would have been much more appropriate for what I shoot. I might just put it in my bag as one of my reserve cards as I don't quite trust it yet.