I used to own that exact model iMac and installed an OWC SSD into it like Mike did above, however I chose to replace the HDD, as I still had a use for the optical drive.
To do this I purchased a 3.5 inch to 2.5 inch drive adapter like this to mount the drive on.
Also there is a propietary Temperature Sensor attched to the old HDD, and if it is disconnected/not connected to anything it will cause the iMac's fans to speed up to top speed (about 4000RPM!).
There are a couple of ways to solve this problem
1) Buy another Sensor similiar to the one already there such as this or this.
(This is the method I used)
2) Purchase/Download software to control and monitor these fan speeds.
A free one is found here: http://exirion.net/ssdfanctrl/
You can also short out the original sensor, but only do this if you know what you are doing.
Here is a guide you may want to check out if considering doing it yourself: https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2308+Hard+Drive+Replacemen t/1766
Any SSD will do the job, however with the new OSX Yosemite making enabling TRIM not exactly possible, you might want to consider a SSD built specifically for Macs-such as the ones from OWC.
Hope this helps🙂