Originally Posted by Star Admiral
The problem is most likely as Balderdash pointed out. Your power supply is inadequate for the graphics card. The Asus card is based off the Radeon 3870 chipset, and those kinds of cards may require up to around 450W of power. Consider upgrading your power supply.
Yeah, this is what I am thinking as well. It sounds very much to me like the new card is using up quite a bit more juice, and it's causing the system to reboot itself because the rig is underpowered.
Unless Astro and the other tech forum regulars have any other thoughts on what it could be...