Hmmm, I found these comments here....
Dual-core: the bottom line here is that due to the threading constraints of the application (ie most of the effort is spent updating the display in a single thread) I do not see much above 60-65% total CPU usage, and I found itís often closer to 50%. So on average, a second core does not show a huge performance increase. Although technically it can be said FSX is a dual-core application, itís much closer to a single-core app on the continuum.
SLI: FSX does not support multi-GPU SLI.