I've posted my settings in the Greystone thread, you can simply load them, and try what's happening on your system.
Note that the AI package I'm using, although it has been made for FS9 (so it might even improve, should the various developers release native FSX models), it's very well done and optimized for fps.
My settings are quite high but, there are some settings you can put very high, without noticeable impact on fps, others that need to be carefully weighted.
For example, under DX9 you shouldn't use Bloom, because it eats up half of the frame rate. At KORD, there's no much need to use Water 2.0, "1.0 high" it's just fine. But, even if you want to use shader 2.0 for Water, I don't see the point of putting too high, the quality increase is slim, but the fps impact is high.
Since we have custom AI ground vehicles at KORD, I simply put the default to 0%. I had 100% (or 50% in the last screen) for airlines and GA traffic only.
Autogen was at Dense (so, 2 notches down from maximum), because I find that it draws more or less the same autogen that FS9 at Extremely Dense. Scenery complexity is at Dense as well, because I can live with the fact that not *all* buildings in Chicago downtown are shown, but "just" most of them.
Settings are meant to be used, depending on the kind of flight you want to do, location, airplane used, weather, etc. They are not a "fire and forget" thing, that you can set once and never touch again. That's probably why MS added an option to load and save settings, so you can create a set of them, depending on the situation.
At Greystone, for example, I can easily crank almost everything to the right, and still getting a good frame rate.
However, your 4 fps looks a little bit suspicious, especially with Vista 64 and 4GB and, especially, with a 3.80 Core Duo processor!! I'm not sure if it's the videocard your bottleneck, before telling you to go out and buy a 9800 (same or better performances than the 8800 GTX, half price), I would check also your whole system config, Windows optimization like turning off unneeded processes and background programs.
For example, since I'm not using the PC for internet, email and basically everything else except flight sim (I'm a Mac user for all the rest), I don't use any firewall or any resident antivirus. If I need to scan a suspicious file, I turn on the antivirus on the fly, then I turn if off.
I also turned off all the annoying Vista background services, like automatic Defrag, Boot optimization, File Indexing, Windows Defender and even the Aero interface...
There's also the issue of several "tweaks" that were commonly used in FS9 and FSX up to SP1 (like the Texture Bandwidth Multiplier), that in SP2, if set with the same settings as in FS9, results in worse performances. You might want to try fresh, moving your FSX.CFG away, and let FSX create one from scratch.