just bought a new Asus A8N SLI with 1 Asus N6600 gt silencer and it keeps crashing when the graphics in my flightsim( CFS3 FS9 ) are set too high. Also a few rally race games crash as well. (DTM2 xpand rally)
No set time period, some times the game freezes and windows is still working in the background ,reset needed,and sometimes it just reboots the PC, sometimes now and sometimes later. If I leave the settings 1 step lower it doesnt crash as often but still crashes, setting 1 step higher and it crashes straight away.
After a crash Temps are ok 53deg graphics and 32 mobo.

A8N-SLI AMD3500 2.2 default setup in bios
2.0gig memory. 2 x 1gig
Extreme N6600GT Silencer/HTD 256mb
81.89 drivers
Win XP pro SP2

OK a BIOS update has stabilized the board so that it only crashes now and again.
Next is to remove windows and reinstall only the updated drivers and no software from the support CD.
Hope that helps.
P.S
you can find a lot of answers when you when you hrs and hrs looking.

sounds like a heat problem, try removing the side of the case and running the game if it works then its a heat problem if not then its either hardware or software related, if it only happens in games then its more than likely hardware related.