What caddy/adapter is needed to install an SSD?
What caddy/adapter is needed to install an SSD into the hard drive bay?
Follow this IFIXIT guide iMac Intel 20" EMC 2266 Hard Drive Replacement. Make sure you peel off the thermal sensor from the HD as you'll need to put it on the SSD.
As for the needed hardware all you'll need here is a 2.5 to 3.5" drive adapter frame.
The other option here is to swap out your optical drive for the SSD. Here you'll need an special adapter. Here's the IFIXIT guide if you decide to go this way: Installing iMac Intel 20" EMC 2266 Dual Hard Drive.