What is the storage capacity of your iPod and what is installed on it besides music?
A movie purchased from the iTunes store will take up 1+ GB of storage - the longer the movie, more storage space required. The new 4S is available with 16, 32, and 64.
In regards to storage capacity, better to have it and not need it than to need it and not have it. Whatever storage capacity you choose, you won't be able to increase it. You probably won't go wrong with 32GB.
Estimate about 4 MBs per song. 100 apps could be around 2 GBs. 100 images maybe a couple hundered MBs. The OS and default apps about 1 GB. That totals around 6.5 GBs. An 8 GB device provides only about 6.5 GBs of free space, net. So you need at least a 16 GB model.