Sarkozy would be in the 2nd round without any problem.
Everything would depend on how the PS would pick its candidate.
If it's a proportional vote in one round, Royal is designated (because Aubry is weakened by Hamon and Fabius). And Royal is beaten by Sarkozy in the 2nd round, although by only 51,5 to 48,5.
If it's a 2-round system, Aubry is narrowly designated. And she may be able to beat Sarkozy in a 1970s-style election, traditional left against traditional right. 50,5 - 49,5.
In this second scenario, there may be an alternative: Royal is candidate outside the PS. Then, Bayrou is able to be ahead of both Aubry and Royal.
And Bayrou is able to beat Sarkozy, 51-49: the first election to be won clearly on the left since 1981..., as Bayrou gathers votes even among Mélenchon or Besancenot voters
(I mean "free" followers of Besancenot, those who voted Le Pen now Besancenot and possibly Bayrou when he is "anti-système"; not the revolutionaries).
So, it would of course be more open than in 2007.
I think Strauss-Khan would be the PS candidate, and his competence makes him able to beat Sarko.