Romney, McCain, Guilliani, Rice, etc. would destroy Hillary
Romney and McCain would be favorites to beat Hillary, but Hillary could still win.
Guliani and especially Rice would lose because they would totally fail to light a fire in the base of the party. Trust me, most everyone I know is Republican and they've confirmed this.
In a general, Guillani is certainly more conservative than Hillary
The Fundies will stay home or vote third party rather than vote for a pro-choice and pro-gay moderate-to-liberal New Englander. If Hillary were the nominee, some thinking ones (most Fundies don't think too deeply) might throw a vote toward Guliani to keep out Hillary, but most will stick with their convictions.
I would actually think that alot of the social conservative block would turn out as would alot of the Republican base. Hillary has been villified by the Republican Party and is a demon of the right since HillaryCare. I think that even with a Giuliani candidacy not only could he appeal to moderates but if Hillary is his opponent he would get alot of the conservative, and especially social conservative vote, since he's not Hillary Clinton and the spectre of her as President would be worse in their minds.