Right - Having a bit of a nightmare here. New 7850 arrived and I did as LtMatt advised above, used DDU to remove the nvidia drivers then had it shut down the PC. Removed 9800GTX and fitted 7850. Started the PC back up and installed CCC and the Radeon drivers. It went through all that fine and then restarted the PC. PC booted to desktop in my monitors native resolution (LG E2242) @ 1920x1080. All fine at this stage. I fired up Arma III and had a go, pleased to see a substantial increase in performance. However, there was some noticeable flickering on the screen, especially in dark/night missions. Anyway, quit the game then went back to desktop. I then installed MSI Afterburner and started that up, however I didn't change anything in there at all. Restarted the PC and it booted to desktop but it was in 1024x768. Tried changing the resolution back to 1920x1080 within CCC but when I do that the refresh rate changes to 29mhz. If I then apply those settings the screen goes black and I get a 'Monitor out of range' message on the screen. I then need to reboot and try again.
I've no idea what I've done wrong but whatever I do I cannot get back to my usual screen resolution even for just Windows, never mind playing a game. I've uninstalled MSI Afterburner but it still does the same. So I uninstalled the ATI CCC/Drivers and restarted then went through the driver install process again. Rebooted and its still the same - 1024x768.
HALP!!!!!