Replace optical drive with SSD?

This series uses a PATA optical drive which I/O speed is 133 Mb/s. Your HD SATA port is SATA II (3.0 Gb/s).

I wouldn't recommend switching out the optical drive for a SSD. Between the very slow I/O speed you'll also encounter issues finding a SATA I (1.5 Gb/s) SSD that will work effectively here.

A better direction here is to switch out your current HD for a SSHD drive like the Seagate Desktop SSHD. That way you gain the speed of the SSD but the storage depth of a standard HD.

The other direction you could go here is putting in a SSD replacing your current HD. Given the age of this system I do caution people from investing too much here. If you find you're only web surfing then a 128 GB or 256 GB (better) SSD might still make sense.