Since that is a Triple Channel board, I would suggest going to 12GB. Either 6 - 2GB sticks or 3 4GB sticks.
You can try matching the RAM you have in the board now - Clock speed,voltage, timings etc, and density (number of memory modules on each stick - and you might get lucky and have that work.
A safer and usually more reliable approach will be to buy a new matched 12GB set, and I would recommend 1333 speed:
Triple channel boards get picky about RAM; you can try 1600 and it probably will work.
My experience was that the board didn't like it and I/we had to send it back and step down to 1333. The performance difference will be unnoticeable between the two speeds.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231356
Pretty good price, too.