i have this rig...

intel q9400
intel dg45id mobo
ati hd4870 pcs+
corsair xms dhx 2gig
wd hdd 320 gig

i'm wondering why is my pc not capable of smooth gameplay in crysis warhead...is the game really demanding?or maybe i don't have fans in my chassis...and also why does my pc have sudden power failures(i.e. restarts suddenly) when it's too hot?. i resort to putting an electric fan in front just to keep this from happening.thanks guys