A generic democrat would probably have an easier time picking Powell, so long as he came somewhat clean about his years with Bush and apologized for that presentation at the UN.
But two African Americans on the ticket would probably not be in anyone's strategy guide. I doubt Powell will take anything more from Obama than maybe an ambassadorship.
But for the sake of argument, let's say both of them consider it. If Mccain gets Powell, the religious right is probably staying home to some degree in this election. They're certainly not donating money (they're saving it for Huckabee in 2012.) Obama probably still picks Biden, since sincere policy wonks who are also capable of holding their own in a debate are hard to come by. Some might say Clark, but you can't beat Powell homeopathically, you've got to hit him from the side.
Mccain's campaign is initially more well-founded with swing voters but significantly poorer. He halts operations in states like Michigan and Wisconsin sooner, but probably has a slightly better shot and Pennsylvania and New Hampshire, not to mention pulling closer in big states that he's not going to win.
Obama victory possibly even more probable, depending on how important you think money is to the campaign process.
If Obama gets Powell, Mccain needs a new VP choice. Palin would be eviscerated by Powell. Two options:
He gets his way and Lieberman goes in. This locks down Florida and given the anger Hollywood felt at Clinton's loss, maybe even some extra funding (but not their vote.) This choice still angers conservatives, but Lieberman's religious enough to impress them, and it's such a strongly pro-Israel ticket that they still give money and support.
Mccain keeps an open mind on economics and is slightly less hawkish on his free market lingo in the days leading up to the banking crisis. Thus he looks less foolish when the market turns into the robot from Lost in Space.
The idea of two African Americans frightens enough swing voters to reconsider Mccain. The popular vote is closer, but the electoral map still shows an advantage to Obama.
The media dubs the Latinos the new soccer moms and both campaigns try to pull them in.
In the end, much closer race. Thanks to Bush, Obama still has the advantage. 60-40 he wins it.
Second Mccain option: Perhaps given Powell's turncoat status, Mccain can guilt Condi Rice into running. First observation is that this should be the most intelligent and interesting VP debate of all time. But in terms of practical effects, this ruins Mccain.
Rice was an outspoken part of trying to implement a two-state solution in Israel. This means Florida starts breaking for Obama sooner. Any state that might be impressed with her record will be more impressed with Powell's. Plus the religious right won't likely be that interested in her. Neocons might love her, but they're on the outs right now. Perhaps a percentage point or two of women swing his way thanks to the Clinton mess, but this choice links Mccain directly to the Bush administration and loses him points in the popular vote overall.
An Obama victory becomes as near to a certainty as can be.