FX or DX
Nikon DSLR cameras now come with two different sensor sizes, DX was the standard size up until the end of 2007, and continues, but in addition FX was added. At the moment the D3, and D700 are the only available FX cameras, with the D3X expected very soon. All the others including the D300 has a DX sensor.
So why select one or the other?
Both FX and DX formats have advantages:-
Both formats are likely to have their following, some insisting that one or the other is the best.
There is nothing magical about 35mm FX format or the DX format, as both came about through a series of accidents or situations, lets look briefly at this. Early silent movie film was unsprocketed and 70mm wide, when they wanted to add sprockets and save film costs they split the 70mm down the middle and added sprockets. 35mm movie film is oriented verticals so images are quite small. This same film material was then turned the other way up running horizontally, so allowing a larger image, and it was decided to advance the film by 8 sprocket holes and allowing for a 2mm gap you have an image area 36mm wide by 24mm high. The 35mm size being the width of the film stock including sprockets. Film is thin and records when light hits it at just about any angle, but electronic sensors are not so thin so more affected by the angle that the light hits the sensor. It was difficult to get a good quality image towards the outside so an area representing about two thirds of the height and width was selected as providing a good image from the central area and this crop size (DX) allowed existing 35mm lenses to be used. DX lenses are smaller as they have to project the image with a smaller diameter, but if these lenses were used with an FX sensor without going to the DX crop then the corners would be dark. The D3 and D700 have an ingenious solution allowing images to be recorded out to the full 35mm or FX format.
The cropping effect that you have with DX lenses means you use wider lenses than you would with 35mm (FX) to produce the same camera angle, and as depth of field increases as wider lenses used, more is in focus at any time with a DX than an FX image. As the image has to be blown up more, a little of this effect is lost. We can calculate this effect, and have tables that allow us to look up the differences. If we take a few examples you will see the difference.
If we put a 60mm lens on a 35mm or FX camera we would get the same image with a 40mm on the DX cameras, so both of these have the same physical image contents and angle, but the amount in focus differs.
If we put a 600mm lens on a 35mm or FX camera we would get the same image as with a 400mm on DX camera.
Depth of field reduction is the largest problem we find professional photographers experience when moving from film to digital as in upgrading they just about always go for longer zoom ranges than they had with film and most don't realise how fast depth of field drops off with longer lenses.
Lenses also get far more expensive as they get longer, so while a Nikon zoom that goes to 400mm is available under £900. The cheapest current Nikon 600mm is £5,499, making FX a far more expensive choice for those looking at using longer lenses. At shorter lengths such as those used for portraiture there is no major difference.
The other possibility for the FX photographer is to add a teleconverter, and a 1.7x produces near the same image size, but at a cost of 1.5 stops of light maybe available to the FX photographer has he can select a higher ISO, and still retain so stopping the f dropping below the point the auto focus will work. This may however not be a solution as according to the specification it appears that the D700 needs a stop more light to auto focus, needing an F5.6 lens, where most others can manage with an F8 lens. You may also have to use non Nikon teleconverters as Nikon teleconverters only connect up to specific lenses. See also article on teleconverters. So if selecting a FX sensor camera, its probably better to budget far more for all telephoto lenses. While cutting the cost of a longer lens, this may not give the depth of field advantage.
The D700 is not greatly different in size to a D300, and weight and size wise the D300 is 825g and 147 x 114 x 74mm while the D700 is 995g and 147 x 123 x 77mm, plus probably lenses will be larger and heavier with the D700. This is going to have little impact on those working in one country but if you fly, then keeping your camera kit and other items you want to carry as hand luggage within your limit is a consideration, or it will have to go in with the other cases and baggage which is not always treated gently and may be lost. You may be able to pay a hefty supplement for extra hand baggage, but often this is not possible. Also with linking flights, often other carriers are involved so while our supplement may cover your UK outward to the first stop, it still ends up in baggage from there on, or you pay again, again, again..... airlines often don't define this, so you may think you have a baggage allowance but this turns out to be only for the first leg.
So who is each best suited for?
The professional photographer doing a wide range of work including studio work, would ideally have both formats, as they both have advantages on some occasions. If selecting one, then perhaps if portraiture indoors was the major focus then the FX would be best as it might be for press work photographing celebrities and for low light sports photography. If landscape, wildlife or longer lens work was the main interest then the DX format would be most likely chosen.
The semi pro, and enthusiast will usually pick one format or the other. Logically and financially the DX format is the best choice for most. There are of course exceptions, including those who like to regularly restrict the depth of field with near subjects, its not a problem with wildlife, birds etc as the depth of field in telephotos is always enough for effects to be used.
Wedding photographers may like the FX format, with its reduced depth of field.
Macro photographers will most likely go for the DX format, as any increase in depth of field makes a great difference.
Wildlife photographers will mostly go for the DX sensor so they get the extra telephoto effect without loosing light.
Dual format on a single camera
The true dual format is perhaps the D3X, not out yet as this will produce a DX image at around 12mp and a larger FX one as well. We don't yet know the price of this.
A D3 or D700 will take images with the DX crop, but at only just over 5 megapixels, and while we would say megapixel counts are one of the less impotent features, at 5 megapixels its a stage most of us left behind a few years back now. So this is a step back.
Bulk and weight
The FX format D3 and probably the D3X when it comes out are large cameras, while the D700 is similar in size to a DX format D300.
Handling dust (vibrator to keep sensor dust free)
FX format the D700 has dust handling, the D3 does not. We don't know if the D3X will have.
DX format the D300 and D60 has dust handling.
ISO (sensitivity setting)
The DX D300 camera goes to 3200 plus 1
The FX D700 camera goes to 6400 plus 2
With most cameras the exposure in the area marked as a plus so many stops usually has more noise, without this the D700 can take images when the light is one stop dimmer. However the D300 can autofocus at F8 where the D700 needs an F5.6 lens, so they will both autofocus in the same minimal lighting conditions.
Active D-lighting is on the FX D3 and D700, and on the DX D300 and D60.
Processing of the images, focussing, live view, rear screen, and most other features are the same on the D300 and D700.
Will one produce better photos?
The simple answer to this is no. A competent well trained photographer can produce the results they require with any of the Nikon DSLR cameras. The aids that do make a difference are things like autofocus, and now all DSLR's have this, aids with tonal range (active d-lighting) available on DX cameras D300 and D60, and FX cameras D3 and D700.
Both the D300 and D700 have very similar controls and handlability and should in theory produce very similar results.
The items you can do to get better photographs in order of effect is:-
Decide what you want to take, if you can, consider the cost of lenses and other items, consider if its going to be taken on flights, and get to see a camera before you decide to buy. Don't be sucked in by the full frame argument, or any of the bigger is best stuff. It needs to be suitable for your use, and the lighter and more compact, as well as cheaper to run, the easier it will be to use.
My current preference out of the full range is the D300, the D700 looks interesting but I don't want to loose either the 1.5x telephoto advantage or the increased depth of field.