What you are trying to do is install a Fusion drive that Apple now sells as an option with the iMacs...a 1 TB or larger hard drive with 256 GB SSD...you cannot do the drive replacement yourself without voiding any warranty you may have on your iMac, and if that does not concern you, just don't break anything because Apple will not help at all if you open it up.
You might try taking it to an Apple store genius bar and if the current drive is bad, see about having them install the Fusion drive and what that would cost. Then you can make some informed decisions.
Also check OWC, http://www.macsales.com and see what they have in the way of hybrids that might meet your needs.
It's over 3 years old and opened up to replace HD before already. Also Apple store won't put fusion drives in old iMacs. Currently they only ship new ones with that drive. So I could just put a 1TB back in but it's a chance to up the performance by adding an SSD for the OS to run on. I just dont know what the set up is, cables etc.
Maybe there is a 'how to' video out there but haven't seen anything yet.
We put a 240GB SSD drive in the CD/DVD drive of my son's 2009 27" iMac. Left the 1TB hard drive in place.
He put OS X on the SSD drive so it now boots from that. He also put the apps he uses the most (Adobe CS6 and Final Cut Pro).
Made a HUGE difference on the speed of this aging iMAC.
We also replaced the hard drive in a 2009 MacBook Pro with a 240GB SSD drive... breathed new life into that as well.