And mismatched memory probably won't run in dual channel
If their identical across both channels they should. He's saying 2 x 1Gb + 2 x 2Gb sticks, this is possible if installed in the following configuration.
(Assuming channel A and channel B, some boards switch the reference up, please read your manual).
DIMM_A1 => 2GB
DIMM_B1 => 2GB
DIMM_A2 => 1GB
DIMM_B2 => 1GB
Channel A => 2 + 1
Channel B => 2 + 1
That can run in unganged mode just fine.