Q: upgrade the RAM snow leopard

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First, check if that's the correct type of memory for the MacBook you are using > http://support.apple.com/kb/HT1651?viewlocale=en_US&locale=en_US Apart from that, you can also see the steps you have to follow to install memory.

Then, you can buy the memory in OWC or Crucial. I recommend one of these providers because their memory is compatible with Macs, but there are different brands that have supported RAM