My E6850, a Core2Duo, that I overclocked from the stock 3.0 GHz up to 3.6 GHz did pretty well. It allowed me to fly in most areas with the graphics at a "medium-high" setting. It did bog down in places like Los Angeles and New York City, but it was still passable. When I added Ultimate Terrain X with it's additional ground vehicle traffic, my dual core CPU struggled a little. Then adding in Active Sky X took a toll on the frame rates, especially when using a FS9 port over aircraft. Then I added Ultimate Traffic 2 and the frame rates took a turn to the south.
Switching to a Q9650 helped me a lot. I found that my particular Q9650 easily overclocked to 3.8 GHz and might go faster, but so far I have not had a need to push it further.
I guess my advice would be that an overclocked dual core is fine with a standard FSX installation, but if you plan on using add-on software, then a quad core really makes more sense.
And I do use a program to assign programs and processes to specific cores. Ultimate Terrain and FSX get to use cores 0,1,and 2, while everything else gets pushed to core 3.