Agree with Al446 here. FSX was not written to take full advantage of the latest multi-core processors.
I use a P4 with 2 GB ram and 256mb AGP Geforce card. An old machine, but it runs FSX with no problems.
As with any other graphic intensive program, I tune the in-game options to get the result I want (an FPS count in the mid 30's). I do not insist on having "all sliders to the right" - just the important ones.
Just checked my install and it stands at 13GB on the HD. I also have some UK VFR scenery which has a size of 8 GB. Hard drive space is useful with FSX. I would not want to run it with much less than (say) 5 - 10 GB free space on the drive.
There is a lot of history / myth / folklore around the MSFS series. Many users have stayed with FS9 and not bothered with FSX, due to their financial investment in FS9 addons.
Now Davy07 can scoff and blow me into the weeds with his super-rig!