A 'Howard' type result (Howard performed above expectations remember - the money appeared to be on a Labour majority of 100+) would see a hung parliament, with Labour as largest party. If the Conservatives take a higher % share of the vote than Labour do, then Labours mandate is shaky to negligable. In such a scenario, Cameron is likely to stay.
With a Hague style result Cameron could go. Only, as Howard chose to do, if there was a potential leader in waiting. There isnt one. William Hague will not take the job, David Davis is still not well liked by MPs etc.
The only potential candidates at this stage in my opinion are Chris Grayling who is already being talked about and is a fantastic shadow minister, and Alan Duncan.
Hague wants to be Prime Minister, when he says otherwise, he's lying. Now, we can be certain he wont take the job again if the situation doesn't look ripe for a victory, but a dissapointment by Cameron may provide actually that.
If the Labour majority increases Cameron will go.
