This effort is like handicapping the baseball season before spring training opens. It makes interesting talk, but it probably won't get enough traction to get anywhere unless you're extremely lucky. I'll take the easy side first.
If the Republicans don't find someone other than their current field, it won't be a race. No one named Bush will be able to win it. The other candidates all either have major baggage to overcome, or they established themselves as non-winners in previous primary seasons. For the conservatives, 2008, looks like a train wreck. However, presidential campaigns have a tendency to find those who lurk in the shadows and turn a spotlight on them.
I expect by the end of 2007, a new unexpected candidate who can say 5 consecutive sentences without an apology or blunder will emerge and lead the pack. My prediction here: no name in the current fray will be nominated unless the Republicans have already conceded the White House.
The Democrats have several motivated high-profile rising stars. As they are exposed to the national spotlight and the grind of a long pre-primary and primary season, the untested candidates may fall by the side of the road. Hillary Clinton can't have much more personal troubles to expose. If she can continue to show the ability to leave her past in the past, she will move quickly to a solid and possibly insurmountable lead. Iowa will really be the test. If they perceive she has abandoned all of her Arkansas experiences, she will fall in Iowa.
John Edwards has just built a mansion in a poor county that he will have to live down. No thinks he can't spend his money how he wants, but a poor man's candidate cannot adopt a rich man's lifestyle this close to the primaries without it becoming an issue. Mr. Obama seems to have a similar problem with a house in Chicago. He is also very untested and could quick rise or fall.
After these three, the waters get murky. I forecast ultimately that we will have our first woman presidential candidate to represent one of the two major parties. It will be Mrs. Clinton against a yet unknown republican. If a strong third party emerges next year, we could see a major move in that direction. Historically, it's not likely, but this seems like it could be the year coming up for history to be made.