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.
Nope, it's managed internally on the card to maintain a temp, if you really don't like the GPU temps use MSI Afterburner to manually control the fan to a higher RPM. This will wear the fan out quicker, but if you want cooler temps that’s the best way.
But in all honestly like the first reply stated you probably won’t see any decreases, especially without spending big bucks on replacing the factory heat sink on the GPU with a water-cooled setup or something similar. Your case fans won’t affect it much.
You are well within permissible operating parameters.