sell D40 and upgrade...the commander mode is one of the most useful feature of the Nikon system...
although some will says can use SB800/SB900 to achieve the same effect but
the built-in commander mode's power is much stronger than SB800/SB900.
a few day ago, I was doing a shoot. one of my fren had a D3 with SB800 set to commander and I was using my D700 with inbuilt flash as commander. The D700 can trigger 2 flashes not in a direct line of sight but the D3 at the same position can't do it!