On my last trip with my D50 and shooting all my images in RAW (2900 when all was said and done) I brought with me 6 2GB cards and 2 of them I had to reformat to use again. Lucikly I was able to download 2 cards to the laptop to free up space. I would have used larger cards, but 2GB is the max size for that camera.
When ever our next trip comes I can't imagine how much space I'll need for my D300. I get about 550 on an 8GB card in 12-bit RAW.
In the end it will depend on how long your trip is, how big the average file is and how many pics you think you'll take. What ever that number comes to figure out how many cards you'll need, then bring at least 2 more above that. Your better off having empty cards at the end of the trip than needing to go though hundreds if not thousands of images to try and free up space with 2 days left in your trip. Each of my last 2 trips I took about 340 images /day.