Azat jan & Edward too -
L'hotel St-Sulpice is very nice, as are number of other boutique hotels in Old Montreal. I'd recommend the Hotel Gault. It's smaller, and has modern minimalist design touches... quite pleasant indeed. Service is impeccable.
Unfortunately, Aznavour's concert happens on the BIGGEST tourism weekend of the year in Montreal. (At the same time as the Canadian Grand Prix) Which brings millions of tourists and their Ferrari's to our city. EVERYTHING is more expensive that weekend. Hotels double, sometimes triple their regular rates, and restaurants hike their prices too. Of course, there's no shortage of fun things to see/do, including free concerts in the street, flashy car shows, fashion shows, street parties, and of course, the required quota of pretty bikini-wearing girls that usually accompany any 'race-related' event.