there is alot of causes
but i have some questions that help in the Diagnosis
do you hear the windows opening? do you see Faint picture or no picture at all?
does the backlight work or black Screen?
does the HDD/CD light blink always...
sorry blizard8 but it look that your MOBO is dieing (maybe not totally dead now)
but for your info
the 670 3.8G CPU run cooler than 560j 3.6G CPU
the 7900GTX run cooler than 6800
i know that cause i upgraded
the new & high...