You could probably get away with a 16GB EASILY if you're not storing music. BUT, don't forget about the camera and the video recorder. The iPhone shoots in HD and the camera can take HDR photos, and those will hog up space quickly if you're one to sync your phone like, once a month. If you don't see yourself using these much, then I'd definitely save yourself the $100 and get the 16GB (unless $100 is chump change, and then I'd get the 32GB for the better resale value). I have the 16GB because I could either afford the 16 or not get one at all, but I sync a couple of times a week and have gotten pretty good at keeping the usage down.
My stats:
Jailbroken via greenpois0n
1158 songs (one of which is over an hour long) at whatever the standard bitrate is for iTunes
401 photos, about 300 being HDR shots taken on my iPhone
27 HD videos with an average of two minutes in length per video
107 Applications, including Infinity Blade and WeatherHD (HUGE files) and a LOT of book apps
My capacity is listed at 14GB, and I have 652 MB available.