I wouldn't recommend going over 1 TB right now, the higher-capacity drives have higher failure rates from what I've heard.
I haven't heard anything like that. Of course, if I get enough I could buy two 1TB drives, but somehow I like the idea of bigger drives.
"I like big bytes and I cannot lie...."
I haven't actually done any price checking in a while, but it used to be that getting higher capacity drives & external enclosure separately was cheaper. This is likely not as true at the size you're looking at, mostly because more people are aware that an external drive is useful

True. The people who market external drives to the "tech-savvy" (or at least as tech-savvy as you can be if you run Windows) have done a good job marketing large drives. I even see some at Fred Meyer (though there's nothing there bigger than 1TB).
I'd recommend putting anything that'll sit around for a while on it (as in, things you won't access terribly often), like Movies & Anime, as well as use it as a backup drive if you don't already have one. Just stick the things you'd be sad to lose (music, those pictures from that trip, those scans that you only have one copy of, saved game files, etc) onto it so that should your internal drive crash you won't be cursing the gods *quite* as badly.
Whichever drive I get will join a growing family. I have a 500GB drive that is almost full-to-the-brim with fansubs. If I want to accumulate much more I need them on a bigger drive. Data expands to fill the available space, and so I need to expand that space so data can continue to safely accumulate.