2 Channel RAM Question

Ok, so I'm running a Core i7 2600k @ 3.4 GHz on a Ausu Sabertooth p67 motherboard, 2x4GB DDR @ 1600 MHz (Corsair Vengeance), and a EVGA nVidia GTX 560 ti with 2GB of VRAM. I'm an Illustration student and I do a lot of work in Photoshop with large images and lots of layers I am also going to be doing a lot of 3d modelling in programs such as Strata and Cinema 4d etc. I run a dual monitor set-up and do a lot of multitasking.

My question is, how exactly does a dual channel system work? will adding another 2x4GB of Corsair Vengeance mean my system will run a little faster(while multitasking, rendering etc.) or because it is a dual channel can it only utilize the the first 2 sticks of ram? Rendering uses a lot of RAM and my understanding is that 3D modelling uses a lot of VRAM but it is also my understanding (could be wrong) that my video card can also use some RAM as additional VRAM. If this is correct it would make sense to me that if I have 16Gb of RAM this will allow the rendering to go faster.

Edit: I should probably add that I'm running Windows 7 64bit.

The computers(Macs, not sure on motherboard or RAM brands) in our lab at school also use the 2600 @ 3.4 GHz but with 16 GB of RAM @ 1333 MHz.

I appreciate any help I can get on this.