Will this RAM work for me? (Late 2009 iMac)
You can find out the memory summary by going to:

 > About This Mac > More Info > Memory



Screenshot example

You can also go to Technical Specifications, click "Browse by Product", and enter the serial number of your Mac to see the specs.

It's always a good idea to read the return policy from the vendors, in case the RAM that you bought was not compatible with your computer.