A fan works for several reasons as I understand it - true for a fan in a room, on RAM, around a heatsink (AC unit, car radiator), for your body, anything.
First it moves the heated air from the area directly next to the source of the heat, making the general area cooler. By moving the hotter air to an area of cooler air, it speeds up the natural process of the Zeroth Law of Thermodynamics by helping the total volume of air reach the same temperature.
By lowering the temperature of the air near the surface of the RAM or its heatsink, it allows the heatsink to dissipate its own heat more easily into that local air. This process is similar to your skin's cooling by sweating. And when you blow a fan on your face when sweating, even though the air around you is equally hot, the process of evaporation works more efficiently and you get cooler. The same process is involved, without the water, for a heat sink with a fan blowing on it.
Even if the source of the air for a RAM fan is hotter than the overall air temp in the case, the processes above will likely overcome it. That is, if you're blowing the hot air rising off the CPU or a hard drive towards the RAM it still gains from the process. This is why a CPU cooler fan that blows the hot air off the RAM across a less-hot area of the motherboard will cool that area off also.
The nature of moving air really doesn't allow the heat to stay close to the chips. The air will find an outlet, cooling the RAM. You can't smother the RAM with its own heat with a fan, the way you might with a blanket.
If you've seen some article that says a fan might do what you said, I'd doubt it and would enjoy reading the article. I'd say it means the author didn't understand much basic science.
From my personal experience it depend on construction of heatsink, plain plate of metal just closing memory to box is not that much efficient as it more prevents air from getting heat out of board and chips. Ones used for example by corsair dominator series are much better and can bring temperature down more then 10 degrees.
In my case it was diference betwen ~60 and 40 degrees i have now (and new memory even run on higher frequency)
It depends really on the construction of the heatsink itself. If its just a little peice of aluminum over the ram most likely it will not be very effective. A couple good sticks with heatsinks are the Corsair Dominator's and OCZ Reapers they are very well designed.
In some cases they are nothing more than paperweights that sometimes fall off and damage things. Some are lightweight and help a little, and some are huge and help a lot. Personally I prefer the mid range, ones that actually pull heat away a little, yet aren't so huge they block your CPU cooler.
heatsinks with fans seem to take up alot of space, i wonder since DDR3 is supposed to consume less power via overclocking at lower levels, and thus emitting less heat, then why are they stacked with crazy heatsinks and fans moreso than DDR2 previously