"If Byrd keels over right before the election and he wins anyways, do they hold a special election to replace him?"
I would assume that the Dem governor would name a Democrat to replace him, and that such Democrat wouldn't face an election until November of 2008 (for the 4 years then remaining in the term). That's what the RATs did in Missouri after Mel Carnahan died.
But if Congresswoman Shelley Moore Capito or (especially) Secretary of State Betty Ireland runs against Byrd, I think they'll win even if he does live to Election Day. See:
http://auh2orepublican.blogspot.com/2005/08/who-is-gops-best-candidate-against.html