As a number of people have said, different people require different amounts of insulation. A silk liner will add a fair bit of warmth to any bag and keep the bag clean, greatly extending the life of the bag and simplifying the cleaning process. Bring a wool cap for the occasional extra cold night.
Construction of the bag and quality of the down is at least as important as temperature and fill power ratings. Cheaper bags have sewn through baffles to stop the down from shifting around. The down doesn't shift, but each seam creates a cold spot. Better bags have box or tube shaped baffles to eliminate the seams and cold spots. Even better bags have "slant tube" baffles, with the tubes shaped like parallelograms so that body heat won't escape straight up the sides of the baffles. The best bags use slant tube construction with a differential cut construction, in which the inner layer of fabric is smaller than the outer layer, helping the baffles retain their shape and keeping the down lofted. Each of these steps is more labor and material intensive than the last, driving up the price of the bag substantially.
Down quality is the other variable, the best down comes from mature birds that have lived outside in the cold for a few years. Cheaper down comes from goslings. Think of fill power ratings and temperature ratings as a way to compare bags from the same maker, but don't expect standards to be too uniform across the industry.
I've found Marmot to make excellent bags (and lots of other stuff), and I've heard very good things about Western Mountaineering sleeping bags. I don't know widely available or competitively priced these names are outside the US, though.
