If it still works, it's a very worthwhile machine - sell it, or keep it as a backup. I've breathed new life into a few of these particular ones with SSDs...should be a great machine for all but the most demanding user.
To answer your question - if I do decide to sell a computer, I usually restore the software to the original factory versions (although now that OS X Mavericks is free, I guess you could leave it on there), and do a secure disk erase just for a bit of peace of mind (I think it's the 8-times option in Disk Utility).