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There really isn't a place to get 4GB of DDR3 cheaper than an direct upgrade from Apple. And it's yet to be seen if third party DDR3 does well with this new revision. (I don't see why it wouldn't though...)
 
Forgive me if this is a stupid question, but dosen't the new macbooks come with 2GB? Is there a reason to buy 2x2GB?

Check the Apple Store and look at the line on the 2GB memory...

--> 2GB 1066MHz DDR3 SDRAM - 2x1GB

Would have been nice to see 1x2GB, but no dice.
 
Just two sticks of RAM (you can't have more than two in a MB anyway)
Each one is 2GB, so it will be 2GBx2GB - it's better if they match. :)
 
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