So, heres my problem. I bought mw2 this week. I put the graphics all on high and resolution to 1400x1050, but it lags alot and runs with ~30fps. My setup is i7 860, 3.6ghz overclocked, 2x ati radeon hd 5750 vapor-x gpus, 4gb Kingston 1066mhz ram, windows 7 64bit, asus p7p55d-e motherboard and MC 620w Carbon power supply.
Update the drivers directly from ATI, you're running crossfire so that makes troubleshooting harder, take one out and see what performance you get, then swap them and make sure their performance is the same, one could be bad.
PS could be under-powering them as well, 620 seems a bit low for your rig.
So when I had only 1 gpu on the upper pcie spot, fps was ~30fps. When I put one on the lower pcie spot, fps goes ~65fps!! What the hell. ;P Both they do same performance when theyre on the lower one and on the upper one.