Although the NDP has pulled ahead of the Liberals and Conservatives in the Québec polling average, they are projected to win only 2 seats, Outremont and Gatineau. The reason is that in 2008, NDP support wasn't strongly concentrated like the Grits' (Montréal) and Tories' (Quebec City). The model assumes a uniform gain across the province, meaning that the projected NDP support is still diffuse.
However, if, as seems likely, NDP gains came mainly in Montréal and the Outaouais, they would also have a shot at Jeanne--Le Ber, Hull--Aylmer and perhaps Pontiac. Additionally, the NDP has a strong candidate in Abitibi--Baie-James--Nunavik--Eeyou. The one other riding where an NDP win wouldn't be completely out of the blue is Westmount--Ville-Marie.
The NDP is therefore looking at up to 7 seats in Québec, assuming that it doesn't rise too far above 25% provincially. (If it does, then anything could happen.) I'd rank these seats as follows in terms of likelihood of win, with main opponent(s) in parentheses:
1. Outremont (Liberal)
This one should be safe for the NDP.
2. Gatineau (Bloc, Liberal)
This seat was a three-way race in 2008, with all parties getting under 30%. The NDP candidate, Françoise Boivin, was a Liberal MP from 2004 to 2006, and lost in close races in 2006 (as a Liberal) and in 2008. The Grits, on the other hand, have a new candidate. All this is quite favourable to the NDP, but a poll conducted in mid-March had Boivin in fourth place. The big question is whether even more Liberal voters than in 2008 will follow Boivin.
3. Abitibi--etc. (Bloc)
I have nothing to say other than apparently, the NDP candidate is popular.
4. Jeanne--Le Ber (Bloc, Liberal)
This riding has always been a tight Bloc-Liberal race; the NDP was a distant third, 19.2% behind the Bloc in 2008. It is a densely populated constituency, mostly working-class, southwest of downtown. Anything is possible here: maybe it'll be a two-horse race again, as strategic voting prevents the NDP from gaining enough. It could be a three-way race. Either the Bloc or the Liberals could lose less to the NDP than the other party, and either end up in a close race with the Dippers or winning comfortably as the other two parties split the opposition. At this point, I wouldn't bet on the NDP, but I wouldn't be surprised if the Dippers do well here.
5. Hull--Aylmer (Liberal)
Liberal Marcel Proulx has a good chance of keeping his seat. The Bloc + Green + NDP vote in 2008 was just 10% more than Proulx's share, so the NDP would likely have to take votes directly from the Liberals if they want a chance.
6. Pontiac (Conservative)
The NDP was fourth in 2008, but only 17.3% behind the winner Lawrence Cannon. It will nevertheless be hard for the NDP to take this seat, which is less urban than Hull--Aylmer and Gatineau, and where the Liberals are running the same candidate that came in second last time.
7. Westmount--Ville-Marie (Liberal)
This is the riding where I grew up, and all my neighbours and friends that lived in the riding were Liberals and New Democrats. (Well, a family from Alberta moved in two houses down. Perhaps they were Conservatives.) It is a Liberal stronghold, and Marc Garneau got more votes than all the non-Conservative candidates combined in 2008. Thus, the NDP would have to take lots of Liberal votes to win. This will be very difficult since there traditionally is no strategic voting in this riding (the Liberal always wins), so the 2008 NDP share likely wasn't depressed.
Overall, I'd say that the NDP has a decent shot at 4 seats. Hull--Aylmer wouldn't be too surprising, but at this point, I just don't see Pontiac and Westmount--Ville-Marie going orange.
Some are comparing the NDP's current situation to the Tories' in 2006, and suggesting that the NDP could well get 10 seats like the Conservatives did back then. However, the Tories benefited from the fact that their new supporters were concentrated in ridings that were formerly competitive between the Liberals and the Bloc. Thus, all they had to do was to replace the Liberals, and presto, le tour est joué.
Where is the NDP most likely to gain votes? Obviously, the answer is the island of Montréal. But most ridings in Montréal are not competitive. This means that in most cases, it is not enough for the NDP to replace the competitive second-place party: they must directly replace the dominant incumbent party. This is a much taller order, and that's why I'd be very surprised if the NDP manages to gain 10 seats like the Conservatives did. It is more likely that the NDP ends up running a (strong?) second in many Montréal island ridings, and a (weak?) second to the Bloc in Montréal suburban ridings.