really depends what you plan on using it for. for some people, like my brother, 8 gb is more than they’ll ever use. they like to have a few CDs worth of music that they listen to over and over. for me, i use almost all 80 gb of my ipod. but i use it completely differently–i load music, videos, and photos. in addition to all that, i often use it as a flash drive. another thing to consider is if you plan on listening more to music or shiurim. 1 shiur will take up significantly more space than 1 song.
good luck!