I'll hazard some estimates: I have a 1:06:05 iTunes bought episode of the Colbert Report. It's standard def and takes 750mb. If you make your own video (ie. download and re-encode or whatever) then you can pretty much get it whatever size you want (and correlating high or low quality) but I estimate that iPad-optimized (576P self-encoded from HD) movies will be roughly 2.5-3GB each.
The only iTunes-bought TV shows I have are kids animations that run 280MB for 24 minutes.
Books are tiny. The article/video that Chupa Chupa linked (top one) suggests that, for now, book publishers are going to sell fancy next-gen interactive, content-rich 'books' as apps, not as books. Color magazines aren't all that huge, either, it's only when they get interactive and animated that you're going to have size issues with literature.
The SD card slot thoughts seal the deal for me and the 16GB unit. It's only for long-haul travel that I'd need more, and then I can load up a few extra movies on an SD card slot and voila, we're golden.
Realistically, with my habits, this'll seldom get far away from my Macs. I don't think it'll type quickly enough for it to replace my laptop for working trips, once I start bringing a dock and power supply and keyboard I might have well brought my laptop, carry-space and weight-wise. Couch surfer that's also great for kids apps, a decent eBook reader, awesome new-book media device, and great digital photo frame/album. That warrants $500 in my family.