Bandwidth caps on T-Mobile vary from plan to plan. The $30/month prepaid option that many people are going with for the N4 offers 100 minutes of voice coverage each month, unlimited text messages, and unlimited data, but only the first 5GB of data is available at HSPA+ 42Mbps speeds. After you exceed 5GB your bandwidth is throttled down to "3G" speeds (but you're not actually cut off).

I think you actually get throttled down to EDGE/2G speeds. I've had this plan for a while and never hit the limit, but I'm pretty sure the limited speeds are slower than 3G.

That's pretty great for streaming music since music can easily be streamed over 3G level speeds. Of course, video is a completely different ball game.

Speaking as a Nexus 7 user (16GB version): I would say that this is the absolute minimum I would have on any device for that matter. 32GB would be some breathing space - for storing films and larger games for example - and would be what I'd go for in any future purchase.

I also have a 50GB Box account on the Nexus 7 which I use (mainly) for pdf storage which helps out a bit.