Dump the 2x2GB you have now and go with 4x4GB sticks. $80 for 16GB is not much of an investment. Seems like you were going to spend that much anyway with 1x8 and 1x4GB.
Or if your board supports it you can probably get DDR-1600 for a few bucks more. But 1333 is fine too. Edit- $10 more for 1600.
Also, I am assuming you are running 64bit version of windows? If not, more than 4GB won't be worth anything.
Edit - Going with single channel ram cuts your ram performance in half. Not ideal at all. If you were talking maybe going with a slightly slower speed it would be one thing, but cutting it in half is pretty dramatic. There are times when more ram is better than faster ram. But why worry about it when ram prices are so good these days?