I used to know all this, but it's been way too long. . .

I have a PC built on the MSI X58 PLATINUM SLI motherboard. Currently I have 6gb of RAM installed (2Gx3|CORS TR3X6G1600C8D R). I'm looking to upgraid my RAM, and since I have 6 slots to play with, I would love to just add some to what I have already. What I'm wondering is whether or not it will be important for the new RAM to be of the same size denominations as the existing RAM. In other words, since I have 3 sticks of 2gb each, do I need to just add 3 more sticks of 2gb each, or can I add 3 sticks of 4gb each, or something of the like?

Aside from making sure they're the same speed, any other considerations I should keep in mind?