I have no first-hand experience, but taking the train looks like a nice option.
The trip takes slightly more than 3 hours, and quoting Via Rail's schedule, there are "five departures every weekday from both Montréal and Québec City, and three departures on Saturdays and Sundays".
It looks like baggage allowance won't be a problem either.
Regarding the price: one-way trip in economy class costs C$85, booked the same day. But if you book in advance, you can get it for as little as $51 on a restricted "escape" fare. More details when you click "Compare" on the booking page. [Prices updated for 2017.]
Edit: The bus option doesn't look bad either. It also takes a bit more than 3 hours from downtown to downtown, and (with a quick look) costs about C$50 if booked the same day. [2012 price.]
One advantage with buses is that there tons of departures every day. For example, Orléans Express, one of the major operators, has some 20 daily departures between 06:00 and 23:00.
To me, both train and bus definitely seem viable options—I guess it's a matter of taste which one you should choose.