You can always use your D50 with any good used flash with auto mode. When I first started out with D70, I used my SB24 flash in auto mode. A bit more troublesome but it gets the job done nicely.
Look around the forums' market place. There are people selling the Sigma 500 DG ST, which is the equivalent of an SB600 in terms of features. The Sigma 500 DG Super is the one equivalent of an SB500. If you're really budget conscience, the ST is alright but then again the SB600 is still a pick...
You can by 2nd hand SB600 or SB800. Or, you can check with TongEh, sometimes he have SB600. He is selling SB800 for $500 (no Nikon warranty, 7 days personal warranty only).
