IIRC you are guessing right. Moon and Sneaksie stated before that the CPU is the limiting factor for fps - not the GPU.
Try to put the graphic settings on "low" or on "high". If there is no significant chance in fps it is the cpu thats limiting it. If there is a major increase when using "low" settings, then your GPU is the limitation.
Depending on the mission and the action going on I always had 10 - 20 fps, but on "high" setting on a AMD X2 4600.
Uwe