For various reasons I am doing a complete reinstall of FSX. I wonder if I should run FSX once after installing, prior before installing SP1, and if I should run SP1 prior to installing SP2... Or can I just as well (or maybe it's even better!?) install FSX + SP1 + SP2 without running the program at all in between? Does it make any difference? Do SP1 and/or SP2 need an fsx.cfg to be there...?

I am not sure about that. Have not managed to run FSX with SP2 and had to reinstall several times. You have to install in a certain order, also the SDK. Every reinstall gave other problems with FSX, so next time I will again uninstall everything concerning FSX and then install every version and try it before installing the next patch. After that install some third-party software.

I found the answer, I think: Phil Taylor said this somewhere else:"after I uninstalled everything, then I would install RTM and run it once to build indices and validate it installed correctly. Once you validated RTM then I would install SP1."So I will run FSX in between installing patches. But I won't change or add a thing!