Garden of Eden, located in the lobby of the Eden hotel at Amstel 144, is a nice quiet upscale restaurant. The menu is somewhat limited, but the quality and service are impecable, and the prices are not outrageous. I always enjoy the mushroom chicken, or the pork chops. Also, the bartenders are friendly and they mix 'em stiff if you know what I mean.
As a side note, the Eden also has my favorite hotel breakfast buffet. More on this in my upcoming 'Breakfast in Amsterdam' thread.