I see the need for lots of storage in the future of anyone who wants to store their media off of the original disks

This seems to be a great drive from what I have read. I got the 500GB My Book for $179, then of course saw it for $169 a week later

802.11n may be fast enough to wirelessly stream audio
reliably, not sure about video. But that is the new standard that they say is really required to do all this streaming wirelessly. In any case building up a good buffer of data that has been streamed is what you want.
As for NAS, I don't like that solution nearly as much as DAS which is basically what you are doing. Network contention on a home LAN and the need for huge throughput do not always get along

Firewire is best if you can do that - less drain on the CPU and just a better protocol. My MyBook is only USB, but I am not using it to stream, just to give me more space.
Fanless drives like this are not new, I have a few. You could easily fry a drive if the internal drive does not transfer heat to the case the way yours hopefully does. This can happen when you buy a cheap external enclosure for one of yuor 3.5" drives and they did not engineer it well

So your case should be pretty warm if it is doing its job...
They will also make some noise because the case also conducts the noise very well!
I will probably do DAS using eSata for my media serving. And you should be able to move your external storage away from your listening area, use efficient heatsinks, very quiet slower fans. How available and convenient this is to us consumers is another question.