I doubt the GDDR5 memory on the HD 6850 would be the problem. I'd trying resetting the BIOS completely (remove the battery for ten minutes), and see if that works at all. Just wondering though, after you removed the drivers, did the computer still work? I.e., when you booted it up would the BIOS and Windows load? And lastly, does your computer work at all? If you put the HD 4650 back in, will it boot up?