actually Mal, the board can hold RAM back, I have a SiS board that I my RAM cannot do 145fsb with and my current Intel chipset board (Asus P4B533) that the RAM will do 155 in.
The board's design can play a part, the way the traces go to the memory can encourage cross-talk at high fsb speeds that will limit stability at high speeds, additionally certain chipsets are more capable at handling high speeds than others, the memory clock generators may be better or other factors may come into play.
The board plays a large part any any OC, the OC can only be as good as your baard, hence the name MOTHERboard.