I've got an SB28 that works fine on my D2x, although it doesn't work in TTL mode. That's not a problem for me because I either use manual (setting the power in increments manually) or in Auto, which is pretty reliable. Same applied to my SB26, an even older flash.
I have an SB28DX, which works fully with my D2x and should work with other digital SLRs from Nikon.
The SB24/25/26/28/28DX/80DX all have PC ports for attaching receivers like Skyports etc. (not sure if the SB50DX has this). Not all of them have optical slaves though to be triggered without an add-on receiver unit - the SB26 does but I'm not sure about the others.
The older SB24/25/26 are bulky, old fashioned affairs with LCD screens that are easy to understand and with slider and button controls for easy use; sliders for mode and normal/rear curtain flash, buttons for f-stops and zooms functions. The SB28 has an all-button interface that's very good, plus it's more compact and much more like the more modern SBxxx flashguns, with the new flashes being very much like mobile phones in their menu systems (the Sb900 is very much like this).
Prices for the older flashes have risen steeply in recent years thanks to the popularity of Strobist.com. I've just paid £50 for an SB24 in good nick; expect to pay £40-£70 for a 24/25/26 and probably closer to £100 for a mint SB28. The DX flashes are probably £100+ used and then it's £150 minimum for the likes of the SB800.
The
Strobist website is probably the No1 resource on the web for info on use, although for technical spec on Nikon flashes I think Mir.com and
Ken Rockwell are worth a look.