I have had the D40 for about 3 years, it was my first digital SLR. I tried shooting with the dial set to "A" and "P" but about 99% of the time I shoot "M" (manual). After talking to my cousin I purchased the Nikon DX 35mm f/1.4. It's was my first prime lens and by far the fastest lens I own. I can't stress how versatile you can be with all those extra stops at your disposal. I still haven't taken the last important step of starting to shoot RAW.
Now onto the camera. Nikon just released some new models, including the D7000, which I really wanted. I went back and forth between the D7000 and the D90. I decided to go with the D90 for now and upgrade to the D7000 soon. Here are the main reasons I wanted do upgrade my D40 to a D90.
1.) Internal autofocus motor.
2.) More than 3 AF points
3.) Higher shooting frame rate
4.) Better CSOM sensor
5.) More dedicated manual buttons - 2 wheels
6.) Larger, higher resolution LCD
7.) Higher ISO capabilities
8.) Video recording
9.) Additional top LCD
10.) Twice the megapixels
11.) Live View Shooting