I know usually it's better to run the memory on dual channel, if possible, with a Kit of 2x2GB, but at the moment my laptop has 2x1gb and I wanted to upgrade it.
Which means, I can either buy a kit of 2x2gb and try to sell the 2x1gb I already have, since the laptop only has 2 slots, or I can buy 1x4gb (also a bit more expensive) and add keep one of the 1gb dimms, making it 5gb non dual channel.
From what I hear, dual channel only gives you performance increase of 3-5%, so I'm unsure of which option to pick.