First I love the older models! But I've got some bad news ;-{ Apple has now listed this model as vintage! So they won't repair or stock parts for it any more. Don't let that stop you here!
OK, so what to do???
If you are leaning towards upgrading your HD or thinking of adding a SSD then you might as well just bite the bullet here and do that now. As to which way to go about it you do have a few different options here: Swap-out the HDD for a bigger HDD or faster & bigger SSHD, or swap-out the optical drive for a SSD drive.
I like SSHD's, or better yet, dual drive setups (HDD & SSD) as you gain the most performance your system can offer. I know this is a bit off from what you asked but you might as well think it out.
Lets say we go with a new HDD or SSHD or install a SSD in the HD bay you'll then need to OWC in-line thermal sensor cable. While some 3rd party houses still have the needed Apple sensor cable you'll need to know who's drive you currently have so you can get the correct one. So if you think your current drive is good enough and you just want to use a dual drive setup this would be the better (cheaper option).