Powell has proven himself a contender in the political arena, whether or not you like him, and has set an excellent example for blacks in federal politics. This means that the middle ground of the USA can now look past race as a factor in elections.
For the extremists, the liberal left will of course see an African American in the set as a plus, while the White Supremacists (or even mere bigots) are generally going to vote for GWB anyway. So there's no net loss there. Where a loss could occur is in the vital Southern Democrat vote, while they might not be against the concept themselves, they might also think that a black man couldn't win and don't want to vote for a loser.
However, there is a major gain when considering the African American vote itself -- they feel horribly neglected by both parties right now (source: NAACP or other African American groups interviews on various news programs I've listened to in the past year) and would feel that an African American appointment would fix this. Plus, it'd eat into the military bloc voting for Republicans.
It would be a big gamble -- if they did this, the democrats would *have* to win in order for African Americans (or even other minority groups or women) to be considered for electing for another couple decades. But I think that we're perched at an excellent time for trying it out. I hope the Democratic Party is running numbers to see how that'd work out.