"Watersports" meaning your some distance away from the action but on firm ground? Or from a boat, closer to the action?? Can you use a tripod???
Well, after all the questions, here are some answers from extrapolating what you'd probably like to to:
First of all: Action + distance spells trouble
! You have a fast moving subject introducing motion-blur
and you'll have camera-shake
due to the long lens you'll need. Fortunately if you shoot fast enough to capture the action, camera-shake will be negligible.
How do you shoot fast action? You need two things: (1) enough light through the lens and (2) a camera with low noise at high ISO (read "sensitivity").
As to the second point (low noise
) this is the major reason, why you need a DSLR because, these cameras are the only ones where you can shoot comfortably at ISO 800 and even higher. This is due to their large sensors, that can capture more light than the puny little sensors of compacts. But even with DSLRs there is a pecking order as to which sensor has the highest sensitivity: Best is FF (read "full-frame" or "VERY expensive"), followed by APS-C (like Canon 400D, Nikon D40, etc.) and that is followed by four/thirds (like Olympus).
At the market segment that you're looking at the Nikon D40 with it's relatively low pixel-count is one of the best cameras for low-noise shots at high ISO and it's also pretty cheap. So you should go for it.
Well the first point (enough light
) is not so easy or cheap to solve. The standard answer is: get a large-aperture lens, "large" meaning the f-number should be as small as possible (confusing, isn't it?). Unfortunately large-aperture lenses are very expensive, so out of reach for a normal budget. The other prob being, that the price of a given aperture (say f2.
increases astronomically the longer the lens is. So here is the ultimate task for you: You have to find out
what the longest lens
/reach is that you need
for your action shots. Because if you can do with a 200mm lens, that will be cheaper (and lighter too) for a given aperture than the 300mm lens.
As I recently startet reviewing tele-zooms
(up to 500mm) let me say that much: 200mm is a real nice length (it is equivalent to 300mm on a film/35mm-camera), at 300mm lenses start to get heavy/expensive (the Nikon 70-300 perhaps being on the better side in both categories) and beyond 300mm it is really getting ugly (price-wise, bulk-wise, shake-wise,...). So depending how close you can get to the action don't look further than 300mm (=450mm film-equivalent), and if you can get a good price for the Nikkor, just go for it (I think though that perhaps
Gordon is just preparing a review of the VR-version of this lens). If you can stretch your budget, go for the stabilized version though: It helps combat shake and let's you shoot at slower shutter-speeds when the action is not too fast-paced!
If you only need a 200mm for action, then report back, because other options arise...
The Nikkor 28-100mm f3.5-5.6G will definitely not help you for distant action, as 100mm (=150mm film-equivalent) is certainly too short and the aperture at the long end is definitely smaller than on the 70-300mm lens. How's that? Well the f3.5 is only valid for say the 28-35mm range and I bet at 70mm this lens is already at f4.8-f5.0 whereas the 70-300mm is f4.0 at 70mm and f4.8 still at 200mm. So in effect the 70-300mm is 1/2 stop faster
at the comparable focal length than the 28-100mm.
: Be aware that you're not going to shoot landscapes
with a 70-300mm though
You need an additional lens/zoom starting at 24mm, better at 18mm
If you want landscape plus action in one
lens you need a 18-200mm!