To explain further...
The 16GB, 32GB and 64GB are referring to STORAGE space, not memory. Storage is like your hard disk drive on a laptop or desktop computer. Since I have a laptop as my primary computer and can store archived photo and video on it, I only got a 16GB iPad. But if it is your only "computing device" and you plan on storing a LOT of photos and video then you might need a 64GB version (you cannot upgrade a 16gb or 32gb with more storage).
Memory on the other hand is NOT upgradeable. It is fixed at 256MB for iPad 1 and 512MB for iPad 2. Most laptops nowadays come with 1GB to 4GB of memory.