Consider using the DataDoubler from OWC as well as the SSD. The things to look at are these:
What drive interface is the HDD's port?
What is the drive interface for the Optical drive's port?
The SSD that you get should not have a faster interface than the port it will be connected to.
For example, if the optical drive port only supports SATA 3.0 Gb/s, the don't bother buying a SATA 6.0 GB/s SSD if you are going to connect the SSD to the optical drive's port.