Manuals for Nikon Lenses

You mean a thing like this? Or the whole manual?
I only have a few manuals from the lenses, and yours I don't have.
But if you don't have special tools I'm afraid you have a nice pile of glass and metal (plastic too) but it won't be a lens anymore.

You'll find manuals for some (though by no means all) Nikon lenses at http://support.nikonusa.com/app/answers/detail/a_id/13948

You'll find manuals there for no less than four different 70-300mm lenses.

Tamron makes lenses with a variety of mounts. Assuming the lens has a Nikon mount, then it is fully compatible with the D200.

Have a close look at the iris ring to see if any think is broking off
or try moving the lens when it is locked on the camera to see if you lose the fee if so it may need a new mount

Is the maximum available aperture dependent on the focus distance? Many macro/micro lenses "dim" slightly as you focus in close.

See if you get 2.8 focused at infinity.

You have named several excellent digital SLR cameras aimed at the top end of the amateur/semi professional market.
I have the Nikon D200 and have been very pleased with it.
As stated by JCDill, you must really decide what you want and can afford. The camera body you choose will commit you down one path or another. For example, if you choose Canon, only Canon or OEM (Other Equipment Manufacturers) such as Sigma lenses will fit your camera body. Canon have a good reputation for their auto-focus, while Nikon make lenses that are the envy of others.
If this is your first digital SLR then you might be best buying an entry level body with a multi-purpose zoom lens such as the Nikon D40. There is a lot to learn.
If you are converting from film SLR and already have a set of lenses, then the cameras you mentioned will probably be able to use your existing lenses (but this is less so with Canon than Nikon). I was ecstatic to disover that my old Nikon 500mm mirror lens works in manual mode (with metering displayed) on my Nikon D200 but would not work properly on my Nikon D70.
I hope this helps.

This lens is too old for the D90, it has not got the electronics for metering and autofocus.
You can use it manually in M mode but you will have to set the aperture and shutter yourself.
The D90 manual has a section on compatible lenses, page 228 in my version.
This recommends CPU lenses for the D90, type G and D.

you cannot "Preview" shots on the LCD screen with the D200. There is no Live View function. Newer models such as D300 and D90 offers such a feature.

Does the camera body has a M/AFswitch? it would be located in front on the left side if that accidently gets changed to M your lens will not AF any more until you change it?? I know it sounds so simple but who knows!! Barry Brown
www.coralreefphotos.com

Sigma lenses sometimes need re-chipping for full compatability with certain Nikon camera. They are usually quite good about doing this gratis.
Try the Sigma on another body at the camera shop or at home (preferably and older AF film camera). If it works then its likely just a chip compatibility issue.