Everyone of us has been through the drill of trying to milk a few more fps out of FSX.
For starters, try setting your frame rate at one half of your screen's refrfesh rate.
As for settings' starting points, don't move any of them further than medium.
Worry about smoothness, not frames.
Google search for Kostas FSX tweaks guide and AVSIM's Guide, those'll give you a variety of ideas as a starting point.
Don't worry about your computer -- none of us have a computer as fast as we'd like to have.