Graphics and lag

I have a nice video card (Gigabyte Radeon HD 6950) but I could only play games like Assassins Creed Revalations and Fallout 3 with low resolution due to my monitor. I went out and bought a Acer S231HL 23" monitor and set up a dual monitor system with my old one. For some reason, when I play ACR or Fallout on it with high OR the lowest resolution I have horrible lag. Can I get any help? I don't know where to post this either.