For the graphics card:
http://www.tomshardware.com/reviews/gaming-graphics-radeon-hd-6870-radeon-hd-6850,2782.html
In the final page of the comparative test there is a chart to compare the GPUs and know starting from which one you'll notice a significant improvement:
http://www.tomshardware.com/reviews/gaming-graphics-radeon-hd-6870-radeon-hd-6850,2782-7.html







