If you want to use Nikon's CLS, you need the SB800 attatched as commander for iTTL to work. The sb 600 can be used as a slave (manual/TTL) but not as a commander. In short, you need at least one SB800 to use CLS, or you can buy the remote TTL commander for almost as much as a used SB800...which i think is lame.

If you don't want to waste money on nikon's TTL system, go SU-4 with vivitar 285hv (or sunpak), with ebay wireless trigger/pocket wizard.

Actually yeah, most bodies can trigger CLS with the built in flash with commander mode, but the D50 and D40 cant.

I dont know what you mean by "automatic"....nikon's "automatic" flash is TTL, you can set the flash to manual slave, where you have to set the flash output power yourself. I'm still waiting for my flash trigger to arrive next week to mess with my flashes