Thats going to depend on your case and how air flows. Try both ways and see which is better. In general pulling cool air in for the GPU to blow out the back makes the most sense, but I have heard of people using side fans as exhaust and getting cooler temps.
most GPU fans are designed for exhaust flow with the fan blowing through the heatsink and into the open air. Im cant see how you have it setup with the fan but I googled a pic of you card and if you can mount the fan on the case (assuming you have a regular atx case) where the PCI-E slot is then have it on exhaust you should be alright.
try to get that heat out of the case with intake in the front and all other fans outward. also make sure your cables are not in the way. This could make a huge difference!
I agree with digitalvivo . . . exhaust, if his assumptions are correct.
We're making a few assumptions here. If you have two other fans, one intake lower front and one exhaust upper rear, then having your new fan blow in may create back pressure on the front intake fan, reducing its important flow. But if you have NO front intake fan, you may get better results making the new fan an intake fan.
Anyhow . . . no need to guess . . . TRY BOTH! Temporarily mount the fan and use CPUID Hardware Monitor to monitor cpu and gpu temps. Use Prime95 to force your cpu to run at max, and pick a gpu stresser to exercise your vid card hard and in a predictable way.