Aspiring photographer with a D300! I read your blog and it seems that you want to keep buying things like books, lenses, and a new camera when you did have a D50 whch is a great camera.(I still use mine) But the best way to improve is shoot everything as much as possible, and learn to use different modes besides auto. There is some great C&C here ,especially from Derrel, so just learn from what people say here and when you go out shooting. I'm no pro ,just a teen with a love for this stuff, but hope this helps.
Oh and I like to think of this quote sometimes:
"...Your camera dosent have a Richard Avedon button does it? Well Avedon sucked, Karsh sucked, Adams sucked, Mary Ellen sucked, Howard sucked, Jarvas sucked. Every photographer in all of history was a horrible photographer for some peroid of time, they learned, they grew, they had dark days, they percevered. That is the way of the artist. Just be patient, keep on going. Transformation takes time from what I seen in my life. It really is worth the wait." - Zack Arais