I would have anther go at the latest version of nhancer and try 175.19 drivers for vista.
follow the directions on here for nhancer
http://www.simviation.com/cgi-bin/yabb2 ... 08959973#5If you prefer no to then try changing these.
You should try reducing your Antialiasing to 8x or 4x, texture clamp=on.
Reduce your resolution in fsx to 1600x1200, try FPS=30, Anti Alisasing CHECKED. (if not using nhancer, fsx antialiasing should be checked).
Mesh resolution, try 38.
Water effect, low2x, or med 2x.
Scenery complecity= very dense
Try these for a Start off.

Q6600 (oc to 3.3ghz), P5KC, 4gb Corsair Dominator, Raptor 150gb, Seagate 500gb, XFX 8800GT