Adding Thunderbolt to iMac for dual monitor display

If your iMac doesn't have a thunderbolt port, it won't work. You'll have to buy the non-thunderbolt version of the display.

If you're unsure about your model, check out the display port on the back. Is there a thunderbolt symbol? There you have your answer.

http://support.apple.com/kb/HT5219?viewlocale=en_US#2

All comes down to if you even have a thunderbolt port. All models (like you said) prior to mid 2011 did not have a thunderbolt port.
Have a look at the port where you plugged in the screen, if there is a lightning bolt symbol there then its thunderbolt, if its a rectangle with lines on either side then it is a normal display port
You will need the cinema display that is not thunderbolt which means you will only have three usb ports on the back and not an ethernet firewire etc.