Its all about size ;-}
Sorry you can't move the current HD over as its a desktop class 3.5" drive where the optical drive carrier can only support a laptop class 2.5" drive.
But that doesn't stop you here! You can put the 2.5" SSD into the optical drive carrier then using the Startup Disk Control Panel select it as the startup drive once you have it prepped with your OS & Apps.
Make sure the SSD you are putting in is able to run as SATA II (3.0 Gb/s) as thats the limit of this system. Some SSD's won't work with a slower system.
You do want to make sure the systems firmware is up to date. follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers before you start.
I also recommend before you start you clean out the junk in your HD and copy off your data onto an external drive so you can migrate the Apps over during the OS install process onto the SSD. You should also run a good cache & log cleaner to get rid of them as well. Here's one you might try: Disk Doctor. This is also a good time to make a backup of everything else onto a external drive.
If you can you might also want to boot up under an external drive to run Disk Utility to clean your HD for any errors running the Repair Disk & Permissions. If you encounter any errors on the drive let us know you may need to think about replacing it as well here.
Now that we are fully prepared to start here's the IFIXIT guide to follow: Installing iMac Intel 27" EMC 2309 and 2374 Dual Hard Drive Don't let the title fool you its the same for either a HD or SSD.
Good Luck with your new speed demon!