Let's all hope that the minimum AMD GPU requirement (HD 5870) is a joke or MANY AMD GPU users will be in trouble. I am afraid if the minimum requirements are to be believed you are better off buying a crappy GTS 450 than having a competent HD 6850.
There's Minimum's, Recommended and then there's Competitive. Personally I have to have >120 FPS and with no Stuttering to be competitive in Multiplayer. So my recommendation as a minimum competitive is a 2GB vRAM GPU and GTX 660 or HD 7850 @ low or no AA, and more competitively a GTX 760 or R9 280X for HD (1920x1080). My best guess is whatever your achieving now take 10%~15% off and that's about where you'll be in Ghosts.
There's a difference between being able to 'run' the game and being 'competitive' in the game.