HI, i have a fx-8350 black edition cpu with gt640 2gb ddr3 graphics.since, now i have the money for a better gpu. I was considering to Upgrade my gpu to a 3gb ddr5 and was thinking to buy a asus gtx 780 ddr5 graphics.SO, my question is CAN i use my old gt640 for onboard graphics and new gtx 780...