well, if you had 2 sticks vs 4 sticks the two sticks would get a better OC....
Even so, i would eb leaning towards the 2x1GB sticks because theres less memory bits packed onto the chip therefore less heat and a higher OC! if you notice in shops, a kit of 4GB (2x2GB) usually has timings of 5-5-5-15 (for 800MHz), whreas a 2GB kit (2x1GB) usually has timings of 4-4-4-12!! that = more speed, however the increased AMOUNT of RAM would make the 4GB the better performance IMO, especially if on a 64 bit operating system.
if 2 sticks will OC better then 4 sticks in general because more sticks means more juice is needed so it will put the mobo in a harder load to achieve the same speed.
its like a car, carrry 2 people and 4 people(sticks of ram),to reach the same speed(800mhz) it will need longer(lower speed) by using the same amount of fuel supply(voltage), if you want to get to that speed using the same amount of time(800mhz) as carry 2 people you will need to press the accelerator deeper so it can provide more fuel(voltage)