New GPU works, but then computer suddenly crashes and won't boot?

Hi everyone. I recently upgraded from a Radeon HD 5670 to a GIGABYTE Radeon HD 7970OC 3GB. It was working fine for the past 10 days or so, then suddenly, last night, one of my displays turned dark gray with light gray lines going up through it while the second display turned black. The audio continued for a moment and then stopped until I had to restart it. When I restarted, the displays said there was no signal. Initially, I solved this by unplugging the computer and holding in the power button for about a minute. However, after I started back up, the same thing happened about 10 minutes later. Rinse, repeat, the same thing happens a third time about half an hour after that, except the screen turns white this time instead of gray. Since then, the computer has refused to boot, with all fans spinning and a single BIOS beep, but the keyboard and mouse don't light up and the displays show no signal.

However, when I plug in my old card, the computer starts up fine. I've also noticed the VGA_LED coming on and staying solid red on the motherboard. Also, I thought initially it might be the power supply, but I changed it this morning with no effect.

Does anyone have some insight into this? Or does the card simply need to be RMA'd? Thanks in advance for any help. Here are my system specs:

OS: Win7
CPU: AMD Eight-core 8120
MOBO: Asus Sabertooth 990FX
PSU: Rocketfish 900W
RAM: G.Skill 8GB 1600
GPU: Gigabyte Radeon HD 7970OC 3GB/Radeon HD 5670