I have 16 GB (2x 8GB cards) in my bag, about 1400 images altogether shot entirely in RAW from a Nikon D80 (10 MP). In day-to-day usage, I can move whatever's on the cards to my computer and then wipe the cards, and since I'll rarely shoot more than a few hundred pics, I rarely fill up even one of those cards. I barely filled both up during a 5 week trip abroad (I actually bought the second one while abroad, got ripped off for it but eh the price of "film"...)
Point is, if you just go out for the day and shoot, odds are you will not need more than 4GB or so for smaller images and 8GB or so for the 20MP+ cameras even when shooting RAW. If you're shooting more than that, you need to learn to shoot less and get more keepers, because no one's going to have 400 keepers in one day's shooting (again, that is if you're just out shooting for the heck of it - professionals, especially wedding professionals, will of course have more shots).