Hi Barry,
The news is mixed I’m afraid...
Yes you can definitely use the pop-up flash on your D90 to wirelessly trigger the SB600 off camera but with the D50 it looks like you will need a flash sync cable.
I’ve just downloaded both manuals from the Nikon site and pages 188 - 190 of the D90 manual show how to set the camera up so that its pop-up flash will act as a wireless commander for up to two groups of off camera speedlights.
In the D50 manual the word “commander” does not appear and the flash modes on page 99 shows just TTL or Manual.
I have to say that I find Nikon's Creative Lighting System to be
hugely impressive. I have the SB800 and three SB-R200s which I mainly use lens mounted for macro work. You might be better off with an SB800 as it can be used as a commander itself. So you could mount it on the hotshoe of your D50 and use it to wirelessly trigger other remotes. It all depends on how deeply you want to get into flash photography (and how deep your pockets are
)
Dave P.