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Should be fine. You will likely need hdd fan control to prevent fans from spinning up. Worth the buck for it.
 
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148834

there is also this "Hybrid" drive but i dont know much about them.

http://www.newegg.com/Product/Product.aspx?Item=N82E16822178380

i have never been inside my iMac so i want to make sure i make the correct purchase. thanks for your help

From my experience the Barracuda line appears to have the 4-pin port needed for the HDD thermal sensor in the late 2009 iMac. My 2009 i7 iMac had a 1TB Barracuda from the factory. I was able to install a 2TB Barracuda in its place. It had the same temp port as the original drive. No fan control software was needed.
 
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