Yup, the good ol' daze of film photography and lenses, lenses and more lenses to do the job needed... The 50mm you have is indeed a good lens for many things, however, for portraits you have to be - in their face - quite a bit, and this can make a few people antsi....
Yes, there is this darn crop factor between digital and film size but don't mistake it for any kind of a telephoto effect, cause it aint..! Your 50 is still a 50, period.. If your dead serious on wanting 'the best quality' for the image you need to find a prime lens, NOT a zoom of any kind, a 1 focal length lens between 80 or 90mm to no longer than 135mm and make it at least a 2.8 or faster. You may want to look into a 90 or 100mm Macro lens in 2.8. Sigma and others have such and they are sharp all thru their focal range from infinity to close up. You would have to just walk in or out to compose your shot.
Nikon made a beautiful lens for portraits, their 105 f/2.5 and it was sharp..! In auto everything lenses these days tho, prime lenses are hard to find and expensive. Technology HAS helped a LOT in contrast and sharpness on zooms so having a zoom lens to fit this bill may not be all that bad..
To say which one would be good for you could be hard and the fact almost any lens today start around $500 and go up fast. Normally you would want a fast lens. F2.8 or better if affordable. The shallow DOF (depth of field) at maximum open aperture would allow selective focusing on the eyes, throwing the back ground and for ground objects (if any) out of focus. Not to mention easier TO focus for you and extra watts of hot lights would not be needed in the studio.
A lens going too long, like a 200mm (and if your too close) will have a flattening effect and the model will not look good, and too wide and you have to get either way too close or you have distortion from the wide effect of the lens.
So, which zooms..? Any of the newer ones with IF (internal focus) ED (low dispersion) glass or the now new nano-coatings would be a good choice. www.dpreview.com has reviews on lenses that may help you choose a good lens. Nikon's 18~200 has quite a zoom spread but max app is 3.5, however, the reviews are not too shabby and it IS a good all purpose lens.
Remember, the D-80 will take any AF Nikon mount lens you wanna throw at it be it a cheaper Pro Master to an expensive Zeiss, however, it is you, the creative photographer than can make or break the image faster than any lens...
Bob - Tucson