As it is an ATA66 drive, I would recommend setting it as a master to one of the optical drives, which becomes a slave.
As it will be storing mp3s (which presumably will be copied to CD), connect it to the same IDE as the optical drive that is not your CD writer (i.e. your DVD player or CD player).
Be careful about where you physically fit it - I found that the temperature of my 3rd HDD became excessive, when I first fitted it right next to an existing HDD. I ended up fitting it in a Coolermaster drive casing in a 5 1/4 bay.