Assuyming it's music only, 8 GB is 8000 minutes (minus some space that's used by the Mac itself, don't know how much that is). Because an mp3 is about 1 Mb a minute. So 8 Gb is 8000 minutes = 2500 tracks = 120 CD's.
Video's take more space. But probably you wouldn't keep a movie on your IPhone once you've seen it 10 times.
I am not a heavy downloader, will the 8 GB last me awhile or should I upgrade to the 16 GB?