Aha, thanks! Crucial's website was very helpful. So it DOES support dual channel memory but according to Crucial I don't have to install in matching pairs.

So if I understand things, I can just fill in the currently empty slots and be OK - but I will get a performance gain by moving things so each bank is filled with identical DIMMs.

Right?

edit: Crucial's site also says that this model Vaio is four banks of one slot each. Just by looking at the motherboard, it looks like two banks of two slots each but apparently looks are deceiving. So I can just install any RAM I like anywhere, I think.