I can tell you my setup at least and see if it gives any suggestions:
Internal:
C:\ = 500GB, Used for my OS and all programs
M:\ = 2TB, Used for Media (TV shows, movies, music, etc)
S:\ = 2TB, Used for miscellaneous storage (MSDNAA software installs, backups of thumbdrives/memory cards, VirtualBox "hard drives", multiple websites, etc)
External:
1TB = Partial Media (M:\) backup (the movies I actually liked! so I can plug it into my new HTPC and watch all the good movies!)
250GB = Purely backup (documents, digital camera pictures, etc)
I now have a 320GB(internal) and a 500GB(external) laying around unused at the moment, but I'm sure I'll find a use for them soon. I would honestly recommend the Western Digital 2TB drives I got to everyone. They have 64MB cache, and 4k block size so you actually get a much faster drive (I can explain that if you're curious, or you can just look it up on WD's site).
Although I haven't used it myself, I would recommend selling off your current drives, getting 3x 2TB ones and put them in RAID 5. That's my future plan at least!