I say Bush still wins because Powell's presence on the GOP ticket would reinforce Bush's appeal to independents and moderates due to Powell's pro-choice stance and also among African-Americans. That said, I think it would flip close states like, at least, Pennsylvania, Michigan, Wisconsin and New Hampshire. And another reason why Bush would win is, of course, his handling of the War on Terror.
I don't know if Powell could also turn states like New Jersey (that Kerry won by just 6 points in real life) and New York into battleground states since a poll released after the 2004 Republican National Convention showed that Kerry was leading in New York by only 6 percentage points (47-41 percent), the moreover that Powell is from New York.