(1) I have been working with 4 GByte cards on my D3 for several years now. I rarely fill them in one shoot, but since they're set to overflow instead of backup filling them is not a problem. I thought about some of the larger cards but since I don't really need them, why spend the money?
(2) Just picked up a D4 and the files are larger so I should probably go to the 8 GByte cards, but the state of the art seems to be 16 and up so that's probably where I'll wind up. I think it's worth using the smallest card you can get away with, even if you have to switch cards once or twice in the middle of a shoot. There's generally some pause during a shoot when you can take care of details like that. It means that if a card goes south you don't lose ALL your shots. I've never had a card fail, but there's always a first time. For important shoots that's what the backup mode on a dual card system is for.