The biggest sd cards I have are 4 GB. My others are 2 GB. Both hold more than enough pictures. The only time I ever had an issue was last fall when I got busy during elk season, and didn't check one of my cameras for about 6 weeks. It was a 2 GB card and it filled up, with just under 3,000 pictures. I can't imagine ever needing more than a 4 GB card, unless you live a long ways from your cameras and can't check them often.
I do have two cards for every camera so I can just swap out cards, and I can look at the pictures when I get home. I do have a digital camera that I always have in my pack, but it doesn't work with sd cards bigger than 2 GB. Which is why I normally prefer to use the 2 GB cards, so that at least I have the option of viewing pictures while I'm in the woods.
The iPhone has a much bigger screen and if there was any way to use my phone to view pictures, it would easily be worth $50 or $60 for me as well.