A McCain/Frist ticket would probably have the best shot in 2008, but a) it's still a long way to go, and b) McCain is unlikely to win the nomination anyway due to his age and a degree of unpopularity among some of the GOP faithful.
Whoever Karl Rove choses to team up with will win the nomination, IMO. Since Dick Cheney will not seak the nomination Im thinkin Bush and Karl rove will back either Frist or Santorum. They fit the neo-con image and agenda alot better than any other possible candidates out there.