Maziar is correct when he says that many will suggest 2X2. The reason that many would suggest that is because it is true. While he may be getting good results from his 4X1 setup, in general, you are more likely to see better results from 2X2. Fewer sticks equals less voltage strain.
Voltage aside, the OCability is a tossup depending on the chips used. I've seen sites that tell you what chips are good and what models/brands/revisions of RAM will have those chips. Some people have an easier time OCing two sticks compared to 4, however, they also say that the larger chips may not OC as high.
2x1GB might get up to 1200mhz on 800mhz ram
4x1GB of the same sticks above might only reach 1000mhz
2x2GB might get up to 1000mhz on 800mhz ram
Obviously these number are made up to demonstrate a point, but that's what I hear on the old 4x1, 2x2 debate.
ha ha ha... I like the way you started your post, szwaba. Voltage aside... lol Consistant voltage is the key here. Yes, better quallity chips matter, but if you had better quality chips and dirty, inconsistant voltage, you would have very poor results. For any given MoBo, your best results will be with 2X2 because it can maintane the voltage better for 2 then 4 sticks. So basically for best results use the best chips and fewer of them. Voltage is everything!!!!