I made a topic here about why my Fallout 3 runs better on Medium settings than low. So it seems that my CPU is a bit of a bottleneck.
Which settings are mainly CPU dependent so I can adjust them?
It's not quite black and white because several settings rely on both your GPU and CPU, but reducing your Resolution, Shadows, and Draw Distance (in your case, the settings on the view distance tab) will net you the most performance if your CPU is the weak link.
If you're using an LCD monitor, as most are these days, then you're pretty much stuck with what your native resolution is for your monitor. Technically, you can run it at a lower res and disable any GPU or Monitor scaling, but most people don't/won't do this.
The two most GPU intensive settings are Shadows and Anti-Aliasing.