Head over to [url]http://www.crucial.com/[/url] where you can choose you Mac Model and it will give you the type of RAM you need for you computer. Crucial.com is a great website to order RAM online.
Yes, I second Christian's recommendation for Crucial.com. Reasonable prices, and have been using them over the past five years with no problems :-)
It's also easy to find the ram that will fit your Mac using their Memory Advisor tool.
If you iMac is CoreDuo NOT Core2Duo then I think you can have a maximum of 3GB not 4GB by the way.