like posted above, no matter how much room you have, you could always need more.
my 1st gen iPhone was 8GB and I got that after having a 16GB Touch for awhile and it seemed at first like 8GB was to small. i quickly learned to adapt and make smart playlists and ended up actually liking manipulating my library to get what i want onto my iPhone. the limited space forced me to be more choosey and i ended up listening to stuff in my library that i hadn't for a long time. because of that limitation, i learned to really love making playlists.
but, that being said, i did end up getting a 16GB 3G. if there was a white 8GB i probably would have just gotten that because i really wanted that color. it is nice having that extra space, but, i understand completely not wanting to spend the extra $100. i had a nice little stash of cash saved up so i figured i'd go large. selling my first gen for $300 helped out to...
i say, if its the kind of thing that will gnaw at you from the inside, and for the next 2 years you'll be kicking yourself for not getting the 16GB, then return it if you are still in the 30 day window. but, if its only a question of space and you'd like to hold on to that $100, you'll be fine with the 8GB. just take some time to make some playlists and swap them out often.
also, the app Simplify is great. It's free and lets you sync your entire iTunes library to you iPhone. it turns 8 or 16 GBs into unlimited GBs.