system restarting while playing game for 5 mins

[walker10]

hello friends ,i have problem which is making me go nuts

my sys config
intel c2d q6600
asus p5kc motherboard
2gb ddr2 800 fsb memory
320 gb sata hdd
xfx 8600 gt gpu
400 watts psu
os=XP sp2

when ever i play games like farcry v1.1,dawn of magic and Brothers In Arms Earned In Blood the system restarts after 5 mins of play,
when the system starts it shows "overclocking failed reset default values"
when i set the reset values it does't boot,
windows loads a little and the system restarts
then i have to open the case and pull the bios battery out and reset the values ,then it works
everytime i starts the game this thing happens
somebody help me please

*games like gta -san andreas and fifa 2005 works fine

 

[AMDSCOrPiO]

psu might not be strong enough

 

[peterhuang913]

400watt PSU definitely not strong enough to drive a C2D and a 8600gt.

 

[walker10]

thanxz very much guys
i guessed so
or might it be the card overheating

how much power will be enough ???

 

[Nick]

doubt the card will be overheating unless its covered in dust get a 550-700W PSU