You can buy a PCI card that will allow you to have extra IDE channels (check Newegg), or if you have a motherobard (or get a motherboard) that supports SATA, then you might be able to use one of your current HDD's and put it on the SATA channel using an adapter, and free up the PATA channel for the optical drive.
ABit motherboards have a Seriell(sp?) adapter that allows this IIRC, but it might only be their higher end boards.