Sapper,
I've always used steel rear sprockets, with no problems. Tusk,Moose etc. nothing fancy.
Chains have been Highest tensile strength I could afford. That not necessarily being the case on the 250 the other guys may be able to give you a better recommendation. I do like the X-ring Idea and just put one on my KX500. The conventional chains take more time to maintain, and the Ringed chains you have to be more careful maintaining.
Are you Racing or Pleasure riding? And What kind of terrain are you spending most of your time on. This might make a difference to someone else giving out some Ideas.
Tuck\o/