Well, stick with the major brands. For 1 computer for just media (music and video,) you should be just fine with a USB drive.
Buy bigger, much bigger, much much bigger than you think you will need. Drives are cheap, and you really don't want extra cables cluttering up the computer space.
For most media that is replaceable, backup is not an issue.
Buuuuut, if the info is expensive or difficult to replace, then consider at least 2 copies of the data.
I always like to make my own externals.
That way I know what goes in them.
Also this way if I decide that I want to make it internal HDD it wont void any warranty I might have
So go with at least 1tb I know you say you have 500gb of stuff but I remember when 1tb first came out and I said wow how could anyone ever have that much stuff. Now I have a server full stuff.
So grab a seagate 1tb...or a 1tb WD green would be good for a external cause then run a lil cooler. And an enclosure that has the interfaces that you desire.
Or if you want something that can do a lot more you could get something like this 2 drive NAS/Server thing