You can use just one stick in one dimm slot, though, it's recommended going with dual channel for better performance.
Also other thing worth mention is all of those motherboards(the ones you listed and the one i suggested) have only two DIMM slots.
check this.. it says it has 4 dimm slots. http://www.asus.com/Motherboard/P8B75M/#specifications
thank you for making the concept clear to me.. just want to ask one more thing if i add 4 gb more with the previously installed 4gb..
is both the rams will work as a dual channel technology ?
or i have to buy dual channel rams together ?