32!
I have zero movies or music but a bunch of games and a small fraction of my photos (6 or 7gb worth) and I really regret getting 32GB instead of 64GB. I really hope the next iPad has a 128GB option.
Granted I buy (and never play) a ridiculous amount of games, but going forward games are only getting bigger. You might never run into a problem with 16gb, it's only kind of a pain to shuffle apps and content around, but if you're going in with the intention to buy games, I think you're in for a much better experience with 32gb.
And if you have a Steam account with more than 20 games you've never played, you should probably get a 64gb iPad.
Edit: You totally changed up that question on me! We'll I'm leaving my answer anyways in the hopes it will prevent more purchases of 16gb iPads!
Laying on the couch and typing is super easy. I wouldn't want to write a novel like that (but I probably could), but chatting/emails/posts to GAF are no sweat. If you're sitting at a desk and typing a lot, you might want to consider a bluetooth keyboard, but I'd figure out your usage patterns first before spending any money on keyboards.