In all likelihood the best FN and LePen can hope for is to reach a second round in the upcoming presidential election. In a second round (possibly against the Socialist contender Ségolène Royal) most conservatives will either stay at home or vote for the Socialists - unless the election takes place under exceptional circumstances, such as riots in the suburbs.
Of course if Le Pen should win the EU is dead. There is no way they could treat France like they ttreated Austria. However, as I wrote above, I don't think there are many europhiles who lose sleep over this scenario. Most unlikely.
That is, unfortunately, a possibility. That LePen could siphon enough votes from Sarkozy to push Segolene Royal (who makes Cindy Sheehan sound like a MENSA member) into the run-off.
What is likely to happen is rather that Sarkozy begins to sound more and more like LePen without the overt racism.
My $.02