i have similar problems playing games on my laptop, however, instead of freezing up it just shuts itself off after extensive game play. it may be possible that your problem is not in overheating but rather something to do with one of your video drivers. check out your video card's website and download any updates. also, search on google for any known problems related to your hardware. from what i have read in a+ books, it is not advisable to take the case off your computer while it is running, that can undermine the air flow scheme and cause it to overheat.
"It's better to burn out, than to fade away."