Heating when playing games

As I play some games, the CPU's temperature gets around 70 Celsius (160 or 170 in Fahrenheit). Fans are around 6k RPM (playing The Sims 3 on Windows 7 Professional, using Everest Ultimate Edition for monitoring). I just wonder if it's ok or not to be like that. BTW, should I put on the hard case while playing? And can someone give some advice for playing games on the MBP? Thanks a lot.