Generally speaking, mainstream video card should be all that you need for getting the best video / movie quality. In the recent past the high-end video cards lacked multimedia subroutines to improve video quality which the mainstream cards did have basically because high-end video cards were more for gaming than anything else. Now all high-end video cards (to the best of my knowledge) have all multimedia subroutines enabled.
Low-end cards are a different story. They will lack some multimedia capabilities to save on cost or power or both. www.Xbitlabs.com did a test on "HTPC cards" to determine which ones offered the best video quality. The cards varied from mainstream video cards like the HD 4650/4670 and Geforce 9600GT down to the low-end HD 2400/3400 and GeForce 8500. The best results were from the HD 46xxx and Geforce 9600GT which tied for first place.