Re: New rig with 260 crashes....
Sounds like memory to me. It could be faulty or incorrect settings.
I'd google memtest86, a memory tester that tests RAM outside of any OS, it just makes a boot disk. Download it, follow the instructions to make a boot floppy or bootable CD, then run it...overnight should be enough. If you can any failures in the tests (takes hours), then revisit your memory settings. Corsair has the Memory man guy on the forums which can help with timings/voltage.
If all clear then maybe try a new bios, reset settings to default and test again. The OS doesn't strain the system that much, that's why it's not cracking up, try opening a large .zip file or copy large amounts of data between 2 drives if you can. That can also stress things and be an indicator.
Also, download a 3rd party tool to check your GPU and CPU temps.
It could also be your power supply feeding the card as well. I had a PSU test fine on a tester and even boot up a system fine but it caused all kinds of instability. Swapping it out with another one fixed the issue.