The fridge should be at around 1.5-3.5^o C (35-38 F), and a 4 star freezer at around -18 to -21^o C (0 to -6 F). Any colder is really just wasting energy. You can get fridge and freezer thermometers to more accurately track the temperature, I wouldn't rely on any indicators that come with the appliance.

If your appliance is older or cheap, it is likely to be far less thermally efficient than a good new model. In Europe the Eurolabel system is used to measure efficiency. Most older Freezers are "E" to "G" rated and use a phenomenal amount of electricity. Newer "A+" rated freezers can use as little as 180 kwh/year, compared with well over 400 kwh/year for less efficient models. The same kind of comparison applies for fridges to a lesser extent.

You can determine the amount of energy your current appliance uses with a plug-in electricity monitor, if you monitor the electricity usage with one of these for a week or two you should have a pretty good idea how efficient your current model is.

Bear in mind if thinking of a replacement that there is some impact to getting rid of your old appliance, you should weigh up the savings (and cost) of a newer model against that impact.