Not all of the iMac's can support a SSD in the optical drive location as these early models had a PATA optical drive instead of a SATA drive.
Then, you need to review the systems specs to see if the HD SATA port is able to be used by the SSD you are looking at. In your case your system has a SATA II (3.0 Gb/s) interface so what ever SSD needs to work at this data rate. One of the problems people face is getting a fixed SATA III (6.0 Gb/s) drive thinking it will play nice in these older systems, sorry to say they don't. You'll need to get a auto SATA port sensing drive instead (one that can run at SATA II).
Cleaning House: But, before you go down this path have you tried cleaning up the systems file system? How about running this first to get rid of the old cache & log files: Disk Doctor.
Make a good backup and then clean off your drive so 1/4 to 1/3 of it is free. You then should look at creating an external bootable drive (ideally FireWire here). That way you can run Disk Utility from it to repair your drive. Sadly you need to do this from an external OS-X bootable drive. If you can't get a FireWire drive you can also do this with a USB thumb drive as well its just very slow. You'll want to fix the disk & the permissions.
The last step here is getting a good file defragmenting tool. I like this one there are others: Drive Genius. While this sounds like a lot, I go though this process over and over again with 100's of Mac's per week. It makes a big difference in performance!
If your drive is having problems: If you still need to replace your HD consider going with a SSHD drive. Here you'll get the snap of the SSD for booting up and the deep storage of a traditional HD.
I would recommend going with a Seagate drive. Here's the drives spec sheet: Seagate Desktop SSHD note the SATA Transfer Rates Supported (Gb/s) line This drive is able to run in a SATA I (1.5 Gb/s), II (3.0 Gb/s) or III (6.0 Gb/s)system.
Don't forget to first make sure your systems firmware is up to date. follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers and lastly here is the IFIXIT guide you'll need to follow: iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement