I just took my own advice and pointed a fan into my GPU from the side.
8° C cooler!!
Yes, definitely. I have a 14 CM fan in the side panel blowing directly onto my GPU. And I have the ASUS EN.GTX580 Direct CU II dual fan and heat-pipe model that takes up THREE slots.
The ambient air temperature around the computer is maybe 65 or 70 F while the temp of the ASUS GTX580 is 147 F (64 C) running full tilt 100% on a project 8033 work unit.
In Southern California I would not go with any less than this level of ventilation for an air cooled GPU.
BTW, the CPU has it's own single fan Corsair water cooler.