Check my PC specs in my signature. I added a Corsair 4GB DDR3 1333 stick today and according to CPU-Z its running at dual channel mode along with my old Dynet 2GB stick. Can this even be possible?
Yes, ... sort of.
Check your motherboard manual. I suspect it will say that your original 2gb and the first 2gb of the 4gb stick will operate in dual channel mode, and that the unmatched odd 2gb will be in single channel mode.
Not to worry, the 2100 is insensitive to ram speeds anyway, and going from 2gb to 6gb will be a big performance plus.
With two slots filled, you should be using dual channel... mostly. The way the chipsets work, I think the odd 2gb will operate in single channel, but I can see how cpu-z will not bother to expand on that detail.