You'll need to look at this article to find your exact model:
http://support.apple.com/kb/ht3011
iMacs from 2008 and 2009 used PC3-8500 RAM, so the memory in my link will not work in your computer. Also, older models had a maximum RAM configuration of 4GB or 8GB (and maximum stick sizes of 1GB or 2GB), so make sure you look into that too.
Here's a Memory Express link for PC3-8500 RAM. They have 2GB sticks for $20 and 1GB sticks for $30 (with limited quantities available).
http://www.memoryexpress.com/Product...)/Default.aspx