Capacity is a personal choice and only you know how many songs, movies, photos, ebooks and apps you intend to use on the device.
If you intend to use the iPad primarily for web use and e-mail and only want to have a modest number of songs, photos, video clips and apps, or price is most important to you, 16 GB should be adequate. In general, however, you likely will be most pleased buying as much capacity as you can afford as it cannot be upgraded after purchase.
Even 64 GB may not truly satisfy those who want to store -- or particularly those who want to create and edit -- hundreds or thousands of songs, photos and videos, particularly given that the high-resolution display dictates the need for much larger image, video and app files than earlier iPad models.
How much content will fit on my iPod or iPhone? (Also applies to iPad)
http://support.apple.com/kb/HT1867
How much space is your Other using? You may be able to reduce.
How Do I Get Rid Of The “Other” Data Stored On My iPad Or iPhone?
http://tinyurl.com/85w6xwn
With an iOS device, the “Other” space in iTunes is used to store things like documents, settings, caches, and a few other important items. If you sync lots of documents to apps like GoodReader, DropCopy, or anything else that reads external files, your storage use can skyrocket. With iOS 5, you can see exactly which applications are taking up the most space. Just head to Settings > General > Usage, and tap the button labeled Show All Apps. The storage section will show you the app and how much storage space it is taking up. Tap on the app name to get a description of the additional storage space being used by the app’s documents and data. You can remove the storage-hogging application and all of its data directly from this screen, or manually remove the data by opening the app. Some applications, especially those designed by Apple, will allow you to remove stored data by swiping from left to right on the item to reveal a Delete button.
Cheers, Tom 😉