You should NEVER color grade on a computer monitor. You need a proper grading monitor (or at least a Panasonic plasma) that has been probe calibrated to Rec 709 specs, fed with a proper video signal coming from an i/o card like an Aja or Black Magic card.
A GPU will not give you anything that is remotedly accurate for video.
Check out the Color and Davinci forums.... this has been discussed there at length... literally hundreds of times in past years.
Good luck!
Richard Cooper
FrostLine Productions, LLC
Anchorage, Alaska
http://www.frostlineproductions.com