Yes including: Automatic Balanced Fill-Flash (with AF Nikkor lenses/Standard TTL Auto including Programmed TTL Auto.
Yes. Be aware that it is a manual flash, so you have to set the power settings manually.
Generally yes, but there are exceptions. It depends on the specific flash.
Actually no they won't. I don't konw alot about canon cameras. but one thing I do know is that it prabably won't work. Just get a Canon flash or a Sigma flash.
Yes it does with non i-ttl auto (2 different f stops) and manual.
Yes it does just got one in the mail today. Tried it out and it works fine.
Yes. The Nikon D40x has a hot shoe on top of the built-in pop-up flash. Be sure to check the compatibility of any external flash unit that you attach to the camera. Note: The question was changed after the first answer was posted, so that the two do not match now. An approach to the new question is that a Nikon D40x is an entry level DSLR, and you may want a flash in that price range. Suitable units are marketed by Nikon, of course, and by Sigma, Sunpack, Vivitar, and Quantary. Features vary with specific models, so you will want to investigate the features-per-dollar for the various units. [spelvin]
nikon or keh
Yes it will. It will work in auto and manual modes. TTL does not work.
yes
the current nikon speedlight range is thus: SB-200 SB-600 SB-800 SB-900 All will work with the Nikon D300s Older speedlights will also work, but without i-ttl mode.
Yes in non i-ttl auto and manual
It works but no auto focus just manual focus.