My sister recently bought an iPhone 4 8GB and she can't get her music or pics on there. She can't even upgrade to the latest iOS. I'd get at a minimum of 16GB but 32GB is best. My iPhone 4 was a 16GB. I couldn't get all my music on there but it worked for me. I now have the iPhone 5s 32GB and love having the extra space. And all my music and photos and apps will fit on there. I have 6.4GB free. You need some growing space.
16GB is the bare minimum unless you plan on only using it as a phone. 8GB is just torturous. The file sizes of apps are ever increasing as well. Put a couple of those on there, a handful of pictures, and some emails...you'll be playing Sophie's Choice with your media very quickly.
32GB seems to be the sweet spot for the average Joe who likes to keep their music library, a decent collection of apps, and pictures without having to clean it out every week.
In the end though, it's all about how you'll use it.