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I'm pretty sure the iMac's use laptop memory sticks, which are quite a bit smaller than desktop sticks. The ones you linked wouldn't work.
 
MacRumors has a guide of places to buy RAM that is compatible with Mac's, which you can view here.

I just bought 8GB of RAM for my late '09 27" iMac from Crucial. I followed their guide to find my Mac and then it linked me to the appropriate RAM. Fast speedy delivery (From the US to Australia) and have had no problems since I installed it.
 
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