Memory Installation Confusion

My Asus P6X58D-E motherboard manual says "Due to Intel spec. definition, X, M, P DIMMS and DDR3-1600 are supported for one DIMM per channel only". (Page 2-11)

The board has 6 slots (2 slots per channel x 3 channels). Does this mean if I use DDR3-1600 memory chips I can only use one slot from each channel? But if I use DDR3-1333 memory chips I can populate all 6 slots?

So, if I buy 6 megs of ram (3 sticks of 2 megs each), I can not put any more ram onto this board. That right?

Thank you,
--
Gritty