Although historically negative advertising has been very effective in many elections, I think the upcoming Obama and McCain campaigns have the potential to change the way elections are run. Not so much because they are virtuous - though I think both candidates are good guys - but because voters may actually have become sophisticated enough to reward a higher road.
This may be too optimistic but clearly Obama's campaign against Clinton appeared to benefit from taking a higher road. When Clinton attacked in debates and in advertising, Obama usually paried rather than fought back. I think many Americans found this very appealing. Clinton supporters feel that she was treated unfairly by press and by the Obama campaign but I think on balance both the Democratic and Republican primaries were dignified and without nearly the amount of negative campaigning there could have been given the years of political and personal baggage from most of the candidates involved in the races.
In terms of Obama vs McCain I think Obama has a huge edge in the sense he can easily lable McCain a Washington insider as he talks about Obama vs McCain's "change" themes. On the other hand McCain cannot label the young and politically inexperienced Obama much except as being young and inexperienced - arguably very desirable traits to many Americans in the current election as long as the candidate is smart and appealing as Obama certainly appears to be to most.