I had a D40 and loved it for years. When the D7000 came out, I gave my D40 to my son and bought it. What an excellent camera. I could have died happy until I got my hands on my daughter's 5D MK ii and discovered the full frame difference in low lights. It was over. I gave my D7000 to my son who gave his D40 to my youngest daughter and $7,000.00 later in lenses and a FX body. I am set.... For now...
One thing to take into consideration, well at least for me. Bodies will come and go. Lenses you buy only once. Buy the best lenses you can afford even if you decide to stay with a DX camera.... For now.. If you get serious, I bet you WILL move to the dark side and buy a full frame. When you do, you will already have some fast lenses. JMHO. But WTH do I know.....