As a number of other posters have pointed out, it depends on what you use it for. If you like music and have a large iTunes library, 8 GB will not be enough. Likewise, movies, pictures and podcasts will fill it easily. If you don't have much to put on it (unlikely), then go for 8 GB.
My personal recommendation would be to get as large a memory as you think you will need - and add to it. So, I'd suggest you get 16GB - at least.
Cheers and good luck