2000 was a low-enthusiasm election, though not to the extent of 1996. The makeup of the electorate with a more exciting proposition is likely to move an even map in Powell's favor. Especially if he can pull 25% of the black vote (or more).
A right-wing 3rd party would shift it back some of the way. But the ability of such a candidate to capitalize on conservative disappointment would be blunted by the establishment sticking with their nominee, not to mention the minefield of race-related missteps such a candidate and its supporters are likely to make.
Considering Bush pushed to the middle and Gore pushed back, I doubt he'd do different here. But with a right-wing third-party candidate on the books, more liberal voters might feel comfortable abandoning Gore (personally I remember it being a soul-searching moment for a lot of people IOTL).
So I think Powell has a fair shot of a victory, but the map is harder to predict. By playing the margins it could be a landslide or fairly close, but I think it's his to lose.
As has been said, you have to probs my make him a widower, and also the likelihood of him winning the nomination is the real question to overcome.
If McCain doesn't run it will help. Also, if he picks W as his VP, then his chances will be a lot better