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Well, most of the stuff I have (media) is on another external hard drive, so the only stuff on my Mac is the usual stuff it came with, Office, Adobe stuff and a few other bits. Around 100gb I think (I have some TV shows on it currently).
 
If you're buying a new external hard drive you'd probably just go with at least 500GB since they are pretty cheap now. That should be plenty of space.
 
I've was given 1.5x as a guideline before I got my TM drive and it works out to be quite accurate. 250GB isn't huge so maybe go for a 500GB drive. 320 would not be big enough.
 
Make it big. I originally used a 320GB as a Time Machine drive for my 250GB internal drive. It filled up pretty quick.

Now I just use SuperDuper! to keep a clone of my drive. If I ever have a problem, I just put the clone in and keep going.
 
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