Shun Cheung , Feb 22, 2009; 06:44 p.m.
May I suggest a different approach?
Since even new D80 bodies are very affordable nowadays, I would suggest upgrading the D40 to a D80, whose pop-up flash can be a Nikon CLS commander, and then you can use the D80 to command multiple SB-600 flashes. The D80 has plenty of other advantages over the D40.
If you don't use an i-TTL commander, you lose all advantages from Nikon CLS.