Watercooling can get expensive very quick if you're going for performance ($200+) and sometimes it's hard to justify the cost with the gains in speed. I justify it by saying that my setup will be one of the most permanent features of my computer. Even if hardware changes, the same watercooling setup will always be there, waiting to be installed for some overclocking. It's also quieter and frankly, more interesting.
Another thing about fans that you should consider is to have the inside of your case be higher pressure than the outside, meaning two fans blowing in, one fan blowing out. This prevents dust from getting inside your case because dust doesn't want to go against a pressure gradient.
OCing is worth it to me. Even if you have stock everything, you can still overclock safely. If the extra performance can be had safely and with stock everything, might as well do it.
As far as the "overclocking big time" thing goes, hehe, our ability to do it "big time" depends largely on the chip and how well it will overclock. Watercooling will definitely get you significant gains over air, but to truly get insane overclocks the temperatures are going to have to get below ambient, and often, way below freezing, as can be had with phase change cooling, not with water cooling.
