Q: How do I remove a memory module?

Pull a little harder on the tab.

Memory Install Video

http://eshop.macsales.com/installvideos/imac_mid_2007_mem_m/

You have some good answers already. Pulling the tab a little harder is a good one but make sure you've untucked the tabs as they are tucked in.

Here is a good article, your model is listed and there are illustrations to guide you.

http://support.apple.com/kb/HT1423