Depends on your room temperature. Mine never fell below 45c when my room was warm during the summer. 66c is ok for short periods of time. If you have at least one case fan, then you have to decide how long you want to keep your cpu and whether spending $30 or more on a premium heatsink is worth it. The stock heatsink is designed to keep your cpu working for at least 3 years.
Thats a little warm for stock speeds, but it really depends on ambient temp in the room. Its not really warm enough to cause problems, but you'd like to see it a little cooler. One thing that might make a difference is the seating of the heatsink. If you don't want to get a new cooler, it might be worth getting a good thermal compound and cleaning CPU and heatsink to remove the old Thermal compound and replace it and make sure its all seated correctly.