Your wasting your time, in all honesty.
Your GPU temps are perfectly fine and WELL within the acceptable limits. You don't need to be concerned with GPU temps until you seem them at least surpass 80!
This extra fan will more than likely not lower your temps AT ALL! You already have 2 intake fans, and 2 exhaust fans, this is MORE THAN ENOUGH air flow.
My advice: Don't waste your $10, it's just not worth it.
EDIT: For proof of this, check what %RPM your fan is running at, unless its running at 100% all the time, which it almost certainly won't be, adding more and more case fans will not help. GPU's are designed to keep themselves cool and within operating temps, that's the whole reason they have their own fan attached.