One of my very favourite hotel downtown is the Vogue/Loews on de la Montagne, and steps away from Ste-Catherine only (many bars & restaurants in the area).
It's very nicely decorated and classy.

You can also try the Hyatt Regency also on Ste-Catherine, although more to the east. It's close to the museum of modern arts and Place des arts concert hall.
http://montreal.hyatt.com/

A lot of people like the Queen Elizabeth also, which is 2 blocks away form Ste-Catherine only, but IMHO, has furniture that better belongs at a retirement home....

If you prefer something inexpensive, there are always the Days inn, Holydays Inn, Marriott Courtyard and so on....
http://www.daysinn.com/DaysInn/control/Booking/property_info?propertyId=04733&brandInfo=DI

Have a nice time in Montreal!
Kisses,
Zoe