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Trying to gauge if it will disrupt airflow is a shot in the dark unless you have the case in front of you. You can imagine every case is set up relatively the same though. My guess is that if the case doesn't already have adequate cooling, go ahead and get it. If it's got 3-4 fans in it already, don't bother.
As long as you have some sort of intake fan and some kind of exhaust fan and they blow the air from one end of the case to the other you should be fine. The only thing a PCI card will do is help keep your video card a little cooler, and in that case I would recommend upgrading the cooling solution on your video card since it will keep the noise down rather than adding 2 more fans. The pci card would be a lot easier to install and cheaper, but it's fun to learn new things. Just make sure you have some kind of jetstream to carry the hot air out of the back of the case while bringing in new air from the front. The pci card is not going to do much for ambient case temperature.