Yes I think you are right. The big airports are the issue with too many facets. These are simply not built within the spec of FSX and in my opinion are faulty. If you get two big airports in the "reality bubble" then watch out. I just enable the airports I'm travelling between if i'm using hi-def airport addons (which is rare).
The fp throughput of the i5 is around 2/3 of the 3960 at the same frequency.
I've just been through the setup of FSX again and in my opinion what counts is pre-rendered frames and a default config. If I go to the GPU control panel and choose 1 max pre rendered frame, then try 2, then try 3, and depending on the memory available, try 4. I just did that and while fps comes down, the steadiness on the eye improves. FSX looks like FLIGHT now whereby I can focus on buildings and count windows. On 1 pre rendered frame it is unsteady.

