I would do it this way:

On the 500GB drive set aside about 100GB for the Operating System.
On the 500GB leave the remainder for your Program Installations.

On the 1TB drive, set aside about 250GB for Backups, like a Ghost Image.
On the 1TB drive, set aside about 100GB or so for your Downloads & personal Data
Leave the rest for Audio / Video files

These values are estimates and can be fine tuned as needed. By doing it this way
the computer will boot from the 500GB drive faster. Also less wear on the 1TB unit.
It will make backups of the System Image easier.