Clinton always ties or gets very close to McCain in polls. But Obama beats McBush (whoops, typo) every time. I will give you my little knowledge on political science below the fold.
Obama tends to do well with polls against McCain. Hillary does not. I would bet he would beat her in a general election. Here is why.
McCain appeals to Fiscal Republicans, but Evangelical Conservatives hate him. But there is one person they hate more than McCain, Clinton. The Evangelical Conservatives would go out to vote for McCain just to vote against Hillary. Now I imagine the Republican Party will make Obama look like a terrorist and such, even with that, the whole base of the Republican party might not go out to vote against Obama. I know some Republicans (Gasp!) and they detest Clinton, but would be okay with Obama in the White House. Plus, when the Republican Party attacks Obama, he will just brush off the attacks as lies, just like he has been doing the last few months.
Note: I am not bashing Hillary, she would be okay in the White House, the problem is getting her elected.