i'm stable with 2 DIMMs... the key to know is...
in bank1 , you can have a single-sided or double-sided DIMM
in banks 2-3, you can only have one doube-sided DIMM, or one single-sided DIMM, or two single-sided DIMMs.
you can not have the equivalent of more than 1 double-sided, or 2 single-sided, DIMMs in banks 2-3, because banks 2-3 are actually shared.
follow the mentioned rules, and you should be fine.