The phone has a quad core processor (lol) which is passive cooled. To make your answer and the story very short, you don't need to worry about how hot it gets.
On a side note, I had mine tested from full power till it went off, playing nfs and it got as hot as....imagine pouring hot water in a can to make a tea and let it cool for 5 minutes. You can barely keep it in you hands...well that's how hot my phone went and yes, I play nfs still daily for about 1 month now, still works like a charm.
Passive cooled BGA's (processors) can go much hotter than a normal cooled component. And if it reaches the extreme it should shut down itself, with another words, heat isn't your issue unless you play NFS for 5 hours directly under the australian 50% sun melting the phone