I've done this, and it got me lower temps to boot.
Also, does your card have the 4-pin fan connector? Then you can get a 4-pin fan, like a PWM fan, and the video card will still be able to control it.
In my situation, I did several tests to check temperatures. You should do the same, warm up the card and turn on the fan and see what temp you achieve when it stabilizes. Then move the fan.
I'm using zip-ties to secure the fan to the heatsink. It's a 120mm PWM 4-pin fan. I spliced my own adapter, which has the tiny video card 4-pin connector on one side, and the larger 4-pin case fan PWM connector on the other. So, it's a nice setup in my mind, where I can easily unplug it from the video card and revert back to stock or some other cooler.
I've heard of other people bending the pins on their video card. You can remove the plastic sleeve that holds the pins and expose the pins, then spread them apart and force them into the bigger connector that you see on the case fans. But I prefer to make my own connector. Note: I can provide pics/details if you want to splice up a connector, or you can search for them. In my case, I had to connect different color wires together because the video card wires used a different color scheme than the case fan wires.
Edit: Also, have you considered just ordering a replacement fan from ebay to toss in your current setup? That's probably the cheapest/easiest solution that maintains "stock" setup.