I have been having texture problems with FSX.  The textures would be clear and sharp at load up and in matter of a few seconds or minutes the textures would go completely blurry.  Other than that the game ran ok.....with no autogen or complexity.  So I upgraded from a XFX Geforce 6800 to a eVGA 7800gs.  Helped some but if I put autogen or more complexity to the scenery it would eventually go blurry and the framerates werent "awesome".  I had been wondering if I was getting all I could out of a 64 bit processer with out the 64 bit OS.  So I downloaded the Windows Vista Beta RC1 x64.  Once I got it all loaded and installed the x64 nVidia drivers for Vista I loaded FSX.  Big difference.  I didnt get to play long but the frame rates were  better, I didnt lose textures, and I could set the autogen and complexity to very dense.  Slight chopiness in the frame rates but nothing compared to XP Pro.  I would say 10x improvement.  These are just prelim results and I will hopfully get to tweak more tonight and see if is a stable fix.

Athlon FX 64 3200+   2.0Ghz
1Gb DDR 400 RAM
eVGA Geforce 7800 GS AGP
Windows Vista BETA RC1

That's to be expected since FSX was designed with Vista in mind. Same thing could be said for Windows XP. I had a FP shooter game "Delta Force Blackhawk Down" that ran like crap on Windows "98". The hardware remained exactly the same. All I did was switch over to Windows XP and suddenly the game ran perfectly. No jerkiness, graphic anomolies or system crashes.