Once you have determined the precise model of MacBook you own, read through this thread:
http://forums.crystalidea.com/viewtopic.php?f=11&t=1073 to find whether others owning the same model have determined the max temperatures for your specific model. Once you know that, you can determine for yourself how quickly you'd like the fan speeds to ramp up below that temp.
It's most likely that the main sources of heat in your machine are the CPU and the GPU (especially if you're hitting 100 degrees C from simple video playback). Determine which fan is closest to which source of heat, enter in what you determine to be the appropriate temperature boundaries for when MFC starts to ramp up the fan speeds, and you're there. Once that's done you can occupy yourself worrying whether you are burning out your fans through overuse vs. burning out the guts of your computer by running prolonged, high temperature generating operations. Welcome to the joys of laptop ownership.
In the meantime, setting your fans' speeds to "Auto" will insure that they run according to Apple's specification for your model. That said, Apple's laptops have a reputation for running hot - it's the trade-off between stylish and compact design vs. speed and capability. Apple packs a lot of processing power into their laptops, but they don't expect the majority of customers to make full use of it, since it is, after all, a portable.
If you do plan to do extensive and intense work on this machine, you might also want to consider purchasing one of those laptop coolers to augment what your internal fans are doing. Capacitors are electrical components which can be adversely affected by prolonged exposure to excessive heat, and there are hundreds, if not thousands, of capacitors in the circuits of your computer. In addition, there are some functions which generate heat, which you may find you don't need active all the time, such as wireless networking, which is always good for a few additional degrees. If you're in one spot that has an ethernet cable, turn off the wireless, and it will save you some heat. Also, wait until your battery is fully charged before doing intense work, as battery charging is also good for several degrees of additional heat.
I've been running Bootcamp on my machines for years, and have found that running Windows is always a hotter affair than running the Mac OS, which in turn led me to this application.
