Ive come to own 4GB of DDR3 1066 memory, my machine currently has 4GB DD3 1600 in it, my question is would i benefit from upping it to 8GB of ram at 1066, or leaving it at 4GB 1600. I use my computer for normal daily use, gaming and occasionally some video editing.
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Memory speed makes little difference in normal desktop usage.
Regarding Quantity: If you are not using all 4GB now, then adding 4 more will also make little difference.