I have an Arctic Freezer 64 Pro and they are great. The heat pipe stock cooler for the opterons is generally pretty good and you won't see much of a difference dropping in a freezer 64 pro. You will see a small cooling advantage by going with a ninja, mine or tunic tower. However, 3'C is about as much as you can hope for.
To get significant cooling results you'll have to go with water cooling, phase change or peltier. Although, there are not exactly for the weak of heart!