On my cubes (4)I replaced all of the fans with these:
http://www.corsair.com/us/cpu-cooling-kits/air-series-fans/air-series-sp120-high-performance-edition-high-static-pressure-120mm-fan.htmlPreviously, I would have them up for about 12 hrs in high clock mode before they would get very hot. Hot enough that the cases were almost untouchable with the middle of your palms. (Sorry, no thermometer.) Yes, I was running them in a lousy space with confined airflow and heat was radiating from the power supplies (Corsair CX 500's.) Now, no problems. I would move them around when I was initially setting them up and a few times they got so hot that some of the o's turned to x's. With these new fans I can stack them, or be lazy about position, and they won't fail. I highly recommend these particular fans. They're a little pricey, but move a ton of air, are relatively quiet, and of very high quality.