Your symptoms point to your PSU being too weak. It's common that when there is not enough power supplied to GPU and CPU the pc will restart.
Check this by running Furmark with extreme burn-in selected. To generate even more load run prime95 or some other CPU stress tool simultaneously with Furmark.
Verify you temps with CoreTemp for CPU http://www.alcpu.com/CoreTemp/
and Afterburner http://event.msi.com/vga/afterburner/overview.htm