Thanks for all the help. Will get on a streetcar at some point definately.
My summary of all your help is:
Walk from the holiday inn on 8th street by either walking to Howard Street and directly to the Moscone.
Walk to Market St from the hotel and then walk down 4th to the Moscone.
Don't walk up and down 5/6/7th.
What about walking down Mission St. How's that?
The intersections of 6th and Market, 6th and Mission and 6th and Howard are all very bad places by San Francisco standards. I don't walk through any of them if I can avoid them. If I had to choose, I would walk down Market St. because there are likely to be other tourists on that stretch (often looking very lost and a bit frightened). On Mission or Howard it will just be you and the crack heads.