I have an early 2008 iMac that overheated all the time, to the point where it would shut down without warning until it cooled off, and this would happen several times a day. I just wanted it to last till the next big iMac refresh, if possible, but that didn't seem likely.
Then I discovered
smcFanControl. (Sounds like a commercial endorsement, huh, except it's free!)
I have it as a login item now, and it runs whenever my computer is awake. I made 2 custom settings for it... one for normal operation, where there' not too much going on, and another for heavy use like converting video formats or burning DVD's, and honestly, since I started using it a few weeks ago my computer hasn't overheated once. It lives in the menu bar, and a single click will raise the fan speeds if I'm doing something that generates more heat. If nothing else goes wrong, I should easily make it to the next upgrade.
You could try turning it on before gaming, and turning it off again afterwards.