Gary ( Reset MCP Alt ) at AVSim has an excellent FSX benchmark and from that has deduced some quite interesting bits about FSX perf on dual and quad cores. Basically if you dont use add-ons, a dual-core CPU is good enough. If you do use add-ons, then a quad-core CPU is likely what you want.

The orignal post is here.

The post announcing his benchmark is now downloadable in the AVSim library is here.

Highly recommended, nice work!