Maybe this is too late but IMHO, the 32GB 3G is the best price/performance to go if you want to purchase the 3G version. I am personally buying the 16GB because I know for sure that I am getting the version 2 next year....
Each people have different usage, but I do not put a lot of movies in here, in fact, I will put 0 movie in iPad and just stream them via Air Video. This is because I have all my movies on MKV format and waiting for 200+ of 4-6gb EACH to convert one by one is very time consuming.
As far as music, I have this special playlists that are around 5GB in size which contains all my absolute favorite music. No, I do not bring all my music collection on the go. These playlists are the same ones I put into my phone...
Photos, I put all my pictures on Picasa and Mobileme... and I have "Web Albums for iPad" apps for accessing pictures on Picasa via iPad.
Unless you really have a lot of money to spend, or planning to put almost every single media you have inside your iPad (which is.... inefficient and begging the question WHY?), buying 64GB will be a waste of $$.
Again, just my personal take based on my personal use