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!