well i have the 16GB
iPhone 4 and i have approx. 1006 songs, 6 5-minute videos, 90 apps, and 125 photos and i have about 1.6GB left for space and i hate it...i miss my 32GB 3GS but there wasn't a 32GB iPhone 4 available at launch where i was.
i have the navigon, tom tom, and magellan app so that takes up approx. 3-4GB right there, so if i didn't have those navi apps...i'd say i could fit 2000 songs at about 5MB/song on my iPhone 16GB with a bit of room to spare.
after reserved OS space and system files, the 16GB really comes to 14GB of usable storage space.