I currently have a Nikon D100. Is it more worth it to buy a new body or lens?

I currently own a Nikon D100 w/ an AF Nikkor 18-35mm 1:3.5-4.5 wide angle lens. I was wondering if it is more worth it for me to upgrade my body to a d5000/d3100 or would it be better if i were to just get a new AF Nikkor 50mm f/1.8D lens? I would mainly like to be able to take quality portraits and candid photography.

The lenses you have can't be used in the auto-focus mode with either the D5000 or D3100

I would suggest you find a good used Nikon D90 or D300. The upgrade from the D100 will be rather amazing and your lenses will auto-focus when used on those two cameras.

I would upgrade your camera to a Nikon D700 because your 18-35mm f3.5 -4.5 lens is a great choice for a full-frame camera. The D700 is one of the best cameras for portraits and candid photography.