It is fairly accepted that a fan at the rear of the case improves the CPU cooling. In this review, HardwareSecrets tests different situations to determine how much a rear case fan can cool down your processor. You can read the results over here.
Here is a snippet:
"It is common sense that installing an exhaust fan at the rear panel of the case helps to cool all the components of the computer, including the CPU. However, most people don't know simple facts about this subject, such as do you need a rear fan if you keep your case open? Do twin-fan tower coolers benefit from a rear case fan as much as one with a single fan? How much better does a high-rpm fan perform than a silent one? In order to answer those questions, we ran some tests, measuring the temperature difference between the CPU core and room temperature in several different conditions."