Replace HDD with SSHD, SSD or optical drive with SSD?

I would think long and hard thinking cloud based solutions are the answer for storing your important files. While I like them they do have there limits. I would still have at least two full backups of everything under my control. There have been cases of peoples stuff being wiped by errors of the storage house, hackers and people making a mistake loosing everything.

I have two external Mirrored HD's (each has two disks). I rotate them every month between home and a bank safety deposit vault. That way if my house burns to the ground I still have a good copy of most of my stuff and anything else more recent I likely have on my iCloud account.

I still have most everything on my systems drives as well or archived on an external drive.

OK, lets move on here:

Sadly, you are at a cross-roads with this system. The optical drive port is PATA (Ultra ATA/100) so its just too slow an I/O port (800 Mb/s) to put either a HD or SSD into it expecting any speed improvement of the system. Your system also has a SATA II (3.0 Gb/s) port so you'll need to be careful what drive you put in as the drive must be able to run at that I/O speed. FYI: Here's a useful tool to convert between Bits (which is what I have referenced here) to Bytes Conversion Tool.

You may want to consider getting a newer system (used) which has dual SATA ports if you are thinking on keeping your system for another couple more years. Given the age and what you have already invested I would leave things as is in this system. If you had your heart of getting more performance you may want to look at the amount of RAM you have, then think about the SSHD. Seagate makes a good one that will work in your system. Do make sure your firmware is up to date first. Follow this Apple TN: EFI and SMC firmware updates for Intel-based Macs.

I would start things off by making sure I have a good backup on an external drive. For now and do some house cleaning getting rid of the junk. Here's a useful tool: Disk Doctor which can clear out the old log & cache files. Once you get that far using an external bootable drive run Disk Utility to fix any errors on your internal disk running Repair Permissions and Disk Repair. Then the last thing here is run a good defragmentation app like Drive Genius.

These basic steps can improve many a system to be as zippy as when you got your system new!