@Kyliesswartz If the fan has to keep working harder and harder, I start to think that maybe it's not keeping up with the demands I'm putting on it, and the computer may soon be hotter than is safe. If you look at your specs (the
advanced ones on apple's website), listed under "Electrical and Operating Requirements" is the maximum recommended operating temperature. That of course doesn't mean that anything above is going to melt your computer, just that there is a point above which high temperatures can indeed start to damage the hardware. This is less likely on macs than PCs as (according to BluebellFlora, who wrote the uninstalling guides that igazor linked to earlier) mac OS has some built-in protective mechanisms, but in theory you could end up burning out your graphics card and even doing irreparable damage to the motherboard. This has actually happened to PC users who don't cap their fps rates while playing TS3, although newer games (like hopefully TS4, but I don't actually know) usually have their own built-in fps limiters.
(Side note: I found out about mac OS's better methods of self-protection when I asked whether I should be doing anything to cap fps, and BluebellFlora's answer was no, but make sure not to let your computer overheat.)
Still, if you hear your fan on overdrive, you might consider quitting the game for a while. Your computer is made of many distinct physical components, after all, and some of them are going to be sensitive to temperature. This is one reason igazor said that playing TS3 could significantly shorten your computer's lifespan; even if you don't melt its insides, the stress of consistent high temperatures will take its toll.