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3GB v 6GB v 12GB - You will see that the difference is very small on games and not noticable by a human.
The tests are of: CAS 8-8-8-24, 1600 MHZ. The only difference is the amount of RAM.
So you can see that the differences are small, even between 3GB and 12GB.
http://www.tomshardware.com/reviews/...de,2264-4.html
So i would go for 2x2 1800/2000MHZ of RAM. And get 2 more later on, or 4 more if you MUST have 8GB.
Remember that you want a good CAS, i would look for something like 8-8-8-24. Remember you CAN clock this if you feel safe doing so.