Most apps don't require much storage capacity for the apps themselves but if an app creates and stores data, you need to consider that.
How many photos do you have stored on your computer and how much storage space is required for the photos on your computer's hard drive?
The same applies to music and video.
In regards to available storage capacity, it is much better to have it and not need it than to need it and not have it - especially with the iPad. Whatever storage capacity you choose cannot be changed/increased later on. Get the most you can afford.