Just an FYI, if you ever want to fill the other two slots, there is no issue with removing the heat-spreaders from any RAM module if you are careful. They really are just for show- at 1.5 or 1.65v, there isn't enough heat there to need anything beyond a little air movement to cool.
Even if you were in need of help cooling the sticks, virtually all heat-spreaders are only attached with some sort of mild adhesive or two-sided tape. No thermal paste or the like. The only real concern when removing them is to take your time and not damage the memory itself. My Corsair Vengeance memory only required about a minute per stick and a credit card to help pry them open; there was no hope for a tower cooler otherwise.