Best way to reduce GPU temp and case temps overall is placing a fan on the rear grill of the video card in the back of the case All the air coming in from the original fan will cool the heatsink and then the heated air will be sucked out the back completely and out of the case instead of lingering in the GPU and causing more heat. And your gpu heat won't go into the case to heat up other components, i did this to my 8800GTS 640mb and temps droped 10C under load . Also You can place a PCI Slot fan below it where the bottom exhuast grills are, both work fine.
I just put a small fan, maybe 30-40mm. It had some other connection so i cut it off and spliced it with a 4 pin connector. And yes it was a real computer fan If you aren't up for this then get a PCI-Slot fan from a store IF you GPU has bottom grills on it.