I want to know which temperature is critical when playing game please.

I5 2.3ghz 6gb Ram

I recently began playing League of legends and my pc start overheating so i decide to download a Temp meter. Actually is in 50* Grades normaly more or less, can someone explain which is good temp and which one is critical for my pc?

I also think i have a lot of dust in my fan so planning to clean.