Karl Rove thought that Bush would beat Dean by a 1980 Reagan margin, if Dean had been the 2004 nominee.
Karl Rove also thought that Romney would beat Obama in 2012, with a minimum of 285 EVs and a maximum of 352 EVs.
Addressing the original question, I'm torn. If we look at this simply as an election of ideals, in which the right beat the left, Dean would presumably lose worse than Kerry because he was even farther left. Factoring in other factors, such as experience, temperament, name recognition, etc, he is also a weaker candidate than Kerry indicating a larger Bush win.
However, something sends me in the other direction. Despite the factors I mentioned in the last paragraph, I think Dean would be a better candidate. Both Gore and Kerry were fairly uncharismatic candidates. I wouldn't call Bush "charismatic," but he was definitely the happier warrior compared to Gore and Kerry in 2000 and 2004. Dean would be infinitely more charismatic, inspirational, etc than Kerry. Perhaps more so than Obama. Dean would be the inspiring candidate, which would probably raise voter turnout a lot. He would do better amongst youth and minorities. He also might turn out a lot of people that don't usually vote or are just apathetic to politics. Dean would have also picked a better running mate, one that would have provided him with both idealogical and geographic diversity and would have used this in an electorally advantageous way.
So my first and my final instinct tells me it would be a Dean victory, with about 51-52% of the popular vote and about 275-290 electoral votes.
