I have been using Nvidia since I got FSX. Started with a 5200 series-went to a 7600 series and now I am using an 8800 GTS. I have never had a problem with their drivers. My current FPS is set at 35 and I get 18-20 on the runway at Tampa-30-35 after I climb out to cruising altitude ( flying mostly prop planes). Also have AMD 5200 Dual core(2.6GHZ)-4GB ram with XP that is only using 3GB.
I found the site listed below and it is full of great Tweaks and settings. I highly recommend it- See below...
TRY THIS---
http://www.highflightsimulations.com/
Found this web site several months ago. Always looking for great add-ons and additional aircraft. They have a new release of the Super Connie( for SP1/SP Acceleration) plus all of the freeware have a setup.exe file for easy loading( no copy paste of files)
Many add-on files for AutoGen and a complete listing of FSX tweaks that has really streamlined my system. They have a section called "must have files" - most are for enhanced effects.They list all the suggested Nividia & ATI graphics card settings as well as suggested FSX graphics settings.They have answered all of my questions the same day, very friendly site!