Q: Can I use an early 20" iMAC with a dear HD as a second monitor on my early 2009 Mac Pro and how to do so

Well, I'm not sure what models of Mac you have. Neither of those listed in your equipment profile are covered in this forum.

However, here are the requirements for Target Display Mode:

Target Display Mode: Frequently Asked Questions (FAQ) - Apple Support

Not the requirement for a newer iMac (27-inch 2008 model isn the oldest supported).

Also. I'm pretty sure the computer used as the second display has to be working.

With a lot of work & luck the screen may be able to be rewired to work as a monitor, but more info needed.

If you don't know the model, find the Serial# & use it on one of these sites, but don't post the Serial# here...

Using this serial number locator is way better than Chipmunk, and more secure:

https://selfsolve.apple.com/agreementWarrantyDynamic.do

http://www.chipmunk.nl/klantenservice/applemodel.html

http://www.appleserialnumberinfo.com/Desktop/index.php

How to find the serial number of your Apple hardware product...

http://support.apple.com/kb/HT1349

If you don't know the model, find the Serial# , but don't post the Serial# here...

Use this serial number locator:

https://selfsolve.apple.com/agreementWarrantyDynamic.do

How to find the serial number of your Apple hardware product...

http://support.apple.com/kb/HT1349