Turn off any filtering or anything working on the display matrix. Stuff like AA, DoF, etc, they use up a lot more resources than they are worth.
I personally think your system is fine, and I know the 550 is the minimum sys req for ghosts, but it should be fine. These games are made to run on consoles, and consoles are far less powerful than the 550ti. From the memory requirements of ghosts I'm guessing they just have fuck huge meshes and textures, so I would suggest more RAM, no virtual memory, and an SSD.
Check out some benchmarks, but I find it hard to believe that the 550ti is choking on ghosts. I've heard the "recommended" gpu is a 780, which is a flagship model. So I'm pretty sure they are just shilling for Nvidia.