1x4gb + 1x4gb ram.. vs 2x4gb ram.corsair vengeance or g.skll ripjaws

Tha's right... As the others sayd, If the RAMs are the same or have the configuration ( the bouth must be the same DDR#, and the same frequency: 1333Mhz, or 1600, or 1866 and so on) they will run together in dual channel like they have bought together. So go on and have fun!

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DIMM = RAM stick

To add to what has already been said, the latency must also be the same. As said, if you buy them separately, make sure to buy two identical sticks, that way it'll work just like a 2x4GB and you won't have to deal with compatibility issues.