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You'd be best off buying RAM that's guaranteed Mac compatible. Both OWC and Crucial are reliable sources for Mac memory.

Crucial's 4GB kit isn't too shabby price-wise, either.
 
I went with 4GB of Patriot Memory from Newegg since it was on sale at the time. I've read that Patriot Memory can be hit or miss, but I've had no problems on my MBP.
 
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