Wow, thank you all for your help! I didn't realize these lenses were fixed aperture - and yep, the FG does have that split ring. I am familiar with those donuts - I don't mind them. Looks like a decent lens to play with provided I can get a good deal, and it's off to save for a larger telephoto. (and another Nikon body which supports the interchangeable screens)
On the subject of larger telephotos which work with the FG, what's out there beyond the 1000mm? What can be had for a somewhat not-wallet-busting amount?
I've never even considered Vivitar for lenses, of those 600 and 800s, what's out there to be wary of/avoid? Certain models with more issues than others?

Thanks again for all your help!