I'm also experiencing such a problem. I've a Full HD(1920x1080) LCD monitor connected to DVI port of HD4850 512MB. Problem is that the display is not clear(some sort of noise/grains appear in the screen). I've tried changing O.S from Windows 7 Ultimate 32bit RC to Windows Vista Ultimate 64bit. No change.
Ah, i too get that, i have to fix it in the control suite.
Mess around with gamma etc.