...or percentage:
57.61%.
I averaged the results of 10 final national poll results, and according to this, Kerry needs undecideds to break towards him 57.61% in order to tie Bush in the popular vote. This essentially means that Kerry's got the popular vote locked up, since everyone's expecting a minimum of 2:1 breakage.
Here's a cool graph showing Kerry's margin of victory if undecideds break at certain percentages:
Much more below the fold.
Here's my prediction:
% Break towards Kerry: 72%
Kerry: 50.21%
Bush: 48.89%
Nader: .90%
And here's an excel spreadsheet for you to play around with.