I have an HD4850 and I was running a dual monitor setup just fine before I went away to school. Now that I'm back for the break when I plug in a second monitor the computer restarts and one of the monitors goes black and the other shows vertical green and white bars. I thought it was overheating so I cranked up the fans and the problem persisted. When I was away, I tried out one of my friend's 6950's in my mobo and installed the new drivers, but afterwards I reinstalled the HD 4850 drivers.
Make sure you remembered to plug the power back in to your 4850 and that the fan is spinning. You can also try re-seating your card and making sure nothing is leaning against it or shorting against it. Also try switching monitors and see if the problem disappears or follows the monitor.