I was going to get a new fan for my video card (to overclock it). Confused on how to install.?

In particular, this (http://www.newegg.com/Product/Product.aspx?Item=N82E16835192007) fan.



But my question is: do you just stick it in a PCI slot and let it cool along with the stock fan, or does it replace the stock fan entirely?|||If you're confused with this, you shouldn't be doing something like overclocking.