If your system is overheating while playing games,then your system does not have adequate cooling.
I've been building my own systems for the last ten years,and one of the things I learned a long time ago is cooling is very important.
A slot fan will help,but I'll bet your CPU Heatsinkfan isn't up to par.Open the case and look...if it's a factory one consider investing $50 dollars in a new HSF,and some Arctic Silver.
If you have an AMD,consider buying a shim so you don't inadvertantly crack the CPU core...they cost about $7-8.
You can get all three at this website,or look around if you decide to do so.
http://1coolpc.com/home/