Is there such a place for quiet dim sum
The restaurant you're referring to is Casa Imperial. The building (The Hood-Gough House) is listed by the City of Toronto as a heritage property and the decor is Victorian with its huge paintings and chandeliers. I think much of the decor was left over from the former Devonsleigh Place.
Their dim sum is pretty good. We recently had dinner there and, as is the trend of many Chinese restaurants in the city, food quality seems to have deteriorated from, say, 6 months ago. Lets hope they don't continue this trend.
Lai Wah Heen and Cha Liu come to mind, unfortnately not in Richmond Hill/Markham. Both are very quiet and without the hustle-bustle of typical dim sum restaurants.
There is a relatively new all-day dim sum restaurant around Woodbine & 16th, in the same plaza as the ramen place. It seemed like a quieter venue as well.