Would a teleconverter work well with a Nikon d80 and 70 - 300 Lens?

Not very well. You need a reasonably fast (f/4 constant or faster) lens to use a teleconverter because the teleconverter effectively reduces the speed (max f/stop) of your lens. Your lens has a max apperture of f/4.5 but needs to be stopped down to achieve decent quality. This will result in around f/8+ when using a teleconverter meaning it will only be useful for tripod full daylight shots. I'll be frank with you and tell you that if you think a teleconverter is a cheap way to get a telephoto you're mistaken. A teleconverter is the only way to make a supertelephoto an even better supertelephoto. Generally you want f/2.8 primes or a really fast zoom. If you're on a budget you can use the nikon 80-200 with a teleconverter to get a 400mm sharp behemoth.

This will result in around f/8+ when using a teleconverter meaning it will only be useful for tripod full daylight shots.

I do have to disagree a bit with this statement. I bought a 1.4 converter for my Canon 100-400mm lens and I rarely use a tripod for anything. I tried a few shots handheld with this combo, at f8 and got great results. Unfortunately, I have to manual focus, which takes a lot of practice. Actually, 90% of my shots of birds are handheld at f8.

I do have to disagree a bit with this statement. I bought a 1.4 converter for my Canon 100-400mm lens and I rarely use a tripod for anything. I tried a few shots handheld with this combo, at f8 and got great results. Unfortunately, I have to manual focus, which takes a lot of practice. Actually, 90% of my shots of birds are handheld at f8.

Hehe, I forgot the "or". In full daylight shooting at f/8 will yield great shots. It's in the afternoon that you'll need a tripod (or monopod for the superteles) or risk getting blur. My 300mm f/2.8 VR (Nikon) disengages VR by default when the teleconverter is attached; however, since the max apperture only drops down to f/4 it's useable in most kinds of daylight. Also, it autofocuses which is imperative for fast moving nature shots (deer, moose, wildcats).

I certainly agree with the part about autofocusing. But, I don't agree re: the part about needing a pod of some sort when shooting in shade with a zoom lens. It takes practice, but steadying techniques can be acquired over time. Here is an example of a shot with my 100 - 400mm lens shot at 400mm, f/8 and just 1/100 sec. handheld. If you figure in the crop factor, I would have been shooting at 560mm. You cetainly don't need good light to shoot at 1/100 second.

Some info that I found: Canon 100-400 weighs 3 lbs while a Nikon 300 f2.8 weighs 6 lbs.

I sometimes shoot hand-held with Nikon 200-400 VR f4 which weighs about 7.2 lbs (don't forget to add another 2 lbs from the camera body). But after a while, you would appreciate the support of some sort ( I do prefer to shoot bird-in-flight hand-held though). Not to mention to get the sharpest picture possible, a tripod does help.

So, whether you should or are capable of hand-holding a zoom lens to shoot perhaps also have to take into consideration the weight of the lens in question ?

Good point, AgingEyes. I never considered that.

I don't think his 70-300 weighs that much. The vr version weighs less than 2 pounds and if not the vr, I'd imagine it weighs less.

I don't think his 70-300 weighs that much. The vr version weighs less than 2 pounds and if not the vr, I'd imagine it weighs less.

True, Frog. 70-300 VR is light. Since fx101 and mm shutterbug were comparing notes, I was simply pointing out something about their lenses.