Ok. Here's the story. I have a Dell Optiplex 380. Two years ago, I bought a Radeon 5570 and it was a simple and easy install. Plugged it in and everything ran beautifully. Now, I wanted more GPU power so I went and bought a 750W Corsair PSU and a Radeon 6870. I've installed the PSU first, and the PC runs normally. I then tried to install the Radeon 6870.
When I plugged it in, and then tried to restart, the power light blinked orange (it is normally green). I then tried to isolate the problem. I unplugged the 6870, and then turned on the PC (it has an onboard chip), and it was fine. I plugged back in my Radeon 5570 and again, it works fine. When I plug in my 6870 again however, the light blinks orange. I've made sure it's securely tight in the slot, I've switched out the 6 pin connectors (my PSU has 4, I tried all combinations), and I've let it sit for about 5 minutes with the blinking lights.
Please help, I have absolutely no idea what to do now.