What's dead? Computer, Graphics card, Monitor?? Powerlight on what? Mac or PC? What's going on?
I'm assuming your computer will not boot with the new graphics card (or old one now). If this is the case, your computer is not dead. I'm still betting on poor seating of the card. Even if you've reseated it 10 times, with graphics cards it will work on the 11th. Try it again.
Also make sure you haven't bumped any other cards out of their sockets. I ruined a remote recording once by not properly screwing in the PCI card of my 2408...
"Why won't this &*$($^@#&*(@#^& thing boot????"
But when do you get the "hard drive failure" message exactly? Maybe you reset bios somehow and it's not reading your drives? We need more details