What is a good speed to make the fan run at to make sure it does not overheat while playing games?

I run it at 50% all the time? Is that an ok speed even when playing games?

Ok so I have a ATI HD 4850 graphics card and 4gb of ram.

First of all, I can play Bioshock pretty well, however not well enough to my liking. I play it on high, and for some reason every now and then the game will stutter. It kind of gets annoying. The game will just quickly stutter then return to normal again. It happens about once every 2 minutes or so. One rare time the game even quickly froze for about 0.5 seconds then returned it normal which got me scared.

Now is this just the game Bioshock itself that does this, or is there a problem with my PC? I really hope its just the game Bioshock which is stuttering, I really hope there is nothing wrong with my PC.

Please help!