Dual Channel Questions

I'm fairly new to dual channel, I will be recieving a Abit KN8 SLI tomrrow. I have 2x256 and 2x512 sticks of ram, my question is can I use one of each in each side for dual channel or am I just limited to using the 512s? That and is dual channel defaulted to being on or do I have to set something in bios or jumpers?
Thanks in advance.

Usually the boards are color coded so that if there are two sticks in two dimms of the same color dual channel will be turned on by default if not then it will run in single channel mode, but if you use all 4 dimms you'll have to loosen up memory timmings or run at slower speeds depending on the chip you have, and it will also make it harder to overclock if you plan on doing that.

good motherboard choice i love it have the 512's in slot 1 and 2 and the 256's in slot 3 and 4 should work just fine