The D3000 series and the D5000 series don't have an internal AF motor, and so rely on AF motors in the lenses, just like you D40. The D7000 series does have an internal AF motor, so it can use all F-Mount lenses, including lenses that don't have their own AF motors. Most new F-Mount lenses have AF motors, but most lenses on the used market don't. You could save a lot of money by getting a D7000 series body and availing yourself of the abundance of lenses on the used market.
The D3200 and later, the D5200 and later, and the D7100 and later, all have 24MP sensors, which is a huge improvement over earlier models.
Each series has a better AF system than the lesser series.
The D7000 series has a faster, better shutter and can use a vertical grip/battery pack, while the lesser series can't.
While a D3X00 and D5X00 might be cheaper to buy, a D7X00 will perform better and can save money in the long term.
- The lens is the thing.
- 'Full Frame' is the new 'Medium Format'.
- "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.