As long as u have 2 pairs installed, i.e. ur using 4 slots, dual channel should still work fine.
(unless ur mobo states that it only uses 2 slots out of the 4 for dual channel, which is very rare)
The mobo will do it's best to equalize the settings of all the memory installed, so it will have them running at all the same access speeds. 128 bit in this case isn't an issue.
If u went to 3 slots, it would definitely go to single.