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.