does the Palit 4870 Sonic Dual and Gainward 4870 GS have much difference?

hi guys,
i have a bit problem here.
Today i have install Far Cry2 and play it for more than 1 hour, then suddenly my pc restart and the screen remain blank. Mean while my GC start to make loud noise due to the fan spinning at high speed. I try to reset the pc a few time but the monitor just wont wake up. Then i switch the GC button to normal mode (Palit 4870 Dual Sonic) After that the pc start up. My question is why this happen and why even when i am gaming the GC fan would not speed up?

yup i have turn off the pc before switch to turbo mode, i have already finish Assassins Creed highest setting with no problem. But now i have prob while playing Farcry2.