It should focus on your D40 no probs. Coming in at around 900 grams,It might feel pretty weighty mind you but weight to me equates to a solid, built-like-a-tank lens. (Like my current 17-55 f2.8) It incorporates the latest silent wave motor so it should simply snap into focus.
Being built for a full frame sensor, you'll need to assess your shooting requirements. On the DX sensor, it'll give you a 36-105mm lens. Naturally on a full frame sensor, it'll be 24-70. The D300 is still a DX sensor whereas the D3 will be full frame. I rarely go under 17mm & most of my work is in the 17-200 mm zone. However, the D300 will likely be around the D200 price when it was introduced...Im thinking somewhere in the $2200-$2400 price. (This will drop after a year or two as did thew D200) The D3, being a flagship model, will be closer to the D2XS...say around $4600-$5000. I'll be getting a D300 as soon as there available to compliment my 2 D200's. As for working on an F or EL, im not sure at this stage. Theres also a very nice looking 14-24 being introduced at the same time as the 24-70 so who knows...maybe you can go 24-24, 24-70, 70-200 if the finances hold up
