capacitors are also used in correcting phase change in a high inductive circuit. However this would have nothing to do with a DC fan. Yes capacitors act as a high pass filter, but there is no AC pass the fan connector and you wouldn't want any AC applied to the fan because it would just lower the fan performance assuming AC existed in the first place.
The easiest way to get a fan to spin faster using a capacitor is to build a Greinache voltage doubler circuit. http://en.wikipedia.org/wiki/Voltage_doubler
but alas, that requires AC as well.
my recommendation is to just get a program that allows you to turn your laptop fan to maximum through windows explorer.
I also came across this http://forum.notebookreview.com/asus-gaming-notebook-forum/641007-g73jh-6970m-upgrade-infomation-27.html
doing some browsing it would seem that they are talking about replacing a capacitor on the 6870m video card to increase the video card's stability at higher temperatures. He also said he was going to keep the card at 100% to keep the card cool as possible. assuming this was the information you were reading, the capacitor has nothing to do with making the fan spin faster.