Hi there, Welcome to the forums
So you're trying to connect a Standard Display Port cable into a monitor and a 6850? I see no issue in that.
For the HDMI and VGA cables it wont work without a converter as VGA is an analogue signal and HDMI is a digital signal. A standard converter cable wont do the job. DVI > VGA and DVI > HDMI works as it supports both signal types.
Even if you purchased the correct converter there is bound to be a loss in quality. In short hand no it isn't advisable to go from HDMI to VGA.
(You can purchase converters for display ports.)