12GB of ram should be plenty. If it isn't, because of the way you're getting 3x4GB sticks to make up your 12, you will still have 3 empty memory slots  to add some more memory at a later date if it needs a bit of a boost.

(Might be something to make sure of when you take delivery of your PC - that there are in fact 3x4gb sticks  installed and not 6x2GB sticks)

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