where i can buy two new memories (2gb each)? i'm from argentina

First we need to know which one of the 9 different models of MacBook you have. To see which model you have go to the Apple in the upper left corner and select About This Mac, then click on More Info and then System Report. When System Profiler comes up check the Model Identifier and post it back here.

And if it's a model 5,2 then whether it's a Early 2009 2.0Ghz or a Mid 2009 2.13Ghz.

Depending on the model MacBooks use three different types of memory.

Try Amazon

The  Store - or if you're handy the job is easy

and the price is cheap from this reliable vendor

I've used for more than ten years.

http://eshop.macsales.com/shop/apple/memory/