Does configuration matter when mixing RAM of different capacities?

they are identical make-model except 2 of them are 1gb and 2 of them are 2gb. is it better to go:

2x1gb and 2x2gb
or
(1x1gb + 1x2gb) and (1x1gb + 1x2gb)

if each channel has a total of 1+2 then as they approach full capacity then each channel will be able to send info to/from cpu on an equal level.

if one channel has 2gb and the other has 4 then once they are at 4gb capacity total only one channel will be able to utilize the remaining 2gb thereby doubling to/from time of those last 2gb

or does it not make a difference at all?
 
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