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Over at Crucial or OWC you can check which hardware (including RAM) is compatible with your machine and buy it.

If you click on the Apple icon in the top left corner of your screen and click 'About this Mac', you should find enough information to get looking on those websites.
Be sure to check out your specs because it's impossible for you to have a 9600M if you bought it over a year ago.
 
You don't have a 9600M if you got it in 2007. You have either an 8600M or an X1600.

Because of this, you can either address 3 or 4GB of RAM. We need to know more about your model.
 
I apologize.

I of course have the 8600m gfx card. Thank you very much for correcting me.
 
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