My trouble is Nvidia video cards and I believe nhancer is the solution. I have nhancer installed but it may not be working. Suggestions appreciated.

Computer: Vista64, Gigabyte UD5 MB, with i7 920 @ 4GHz, 6BG DDR3 1600 ram, 7200rmp 500GB hd, 30" monitor @ 2560x1600 res.

I've been able to get good results with an ATI4850 card, but I prefer using my more powerful Nvidia 8800GTX.
* After uninstalling ATI card and installing Nvidia drivers and 8800GTX, I installed nhancer 2.4.2. Since Microsoft Net Framework 2.0 wouldn't install on my 64 bit system I instead installed Net Framework 3.5.

With the MS Flight Simulator X profile highlighted I made all of NickN's suggested changes, but the computer ran very jerky with artifacts and sometimes very low frame rates. Obviously I didn't do something right. I tried turning antistropic filtering on and off but saw no results in MSFS X.

Questions:
Do I need to shut down MSFS X and restart it every time I make a change in nhancer?
Does nhancer work immediately or do I have to rebook my computer?
What activates changes in nhancer so that MSFS X is affected by these changes?
Is Vista64 supported by nhancer? The help page for nhancer doesn't list Vista as a supported operating system, but that support page may be in need of update (it talks about this "new" concept called SLI).
thx!

NickN, your comments are helpful. None of the video cards are overclocked, so I will concentrate on the other issues.

The plot thickens...
My Asus board has a FSX install which is now able to run either ATI or Nvidia graphics cards well. I haven't tried nhancer with it yet but I will. My Gigabyte board has two separate Vista64 installs on two hard drives, and two completely separate FSX installs. One install is horrible with both ATI and Nvida, and the other is more-or-less okay with ATI but not so with Nvidia. Looks like I might have a bad FSX install on one hard drive and there's also the possibility of a hardware issue which needs to be resolved. I loaded FSX and Acceleration on both (using two different FSX DVDs, one of which is Gold), and I did not load any service packs between FSX and Acceleration installs, so that's not the problem.

I'll report back.

NickN,
You may have saved me. What I thought was Nvidia/nhancer blues might in reality be bad install blues. I installed FSX and Acceleration one right after another on the Gigabyte hard drives, and I bet that's what's messing up my video. I'll reinstall using your suggested technique.

As for the Asus board, let me clarify. First I was running an ATI video card and getting excellent smoothness. I then uninstalled the ATI card and installed an Nvidia card, and I'm getting smooth performance with that card as well.

Will report back. Many thanks for the info.