The first point is that the 40GB is reduced to around 37GB in terms of available space-the difference is due to utilising the software etc to make the thing run.
My 40GB iPod is full and it holds 9163 songs which is around 28 days of continious music using AAC.
My 40GB iPod is full and it holds 9163 songs which is around 28 days of continious music using AAC.