Hi
Nice to hear you got everything together

Sorry to hear of your disappointments though

A lot of it is your graphics card. I had a feeling you might not be too pleased with the performance of the 9800gtx. While it is a fair card, it is quite a bit below the top running single core cards. You can probably get a few more fps through more tweaks, but in all honesty, the 9800 just isn't really powerful enough for fsx.
Are you using the 3 monitors you wanted to try, or just one?
If it isn't too late, I would return it for exchange on a 280 or 285 gtx. Another option would be to get another 9800 and run in sli mode, but there are still some issues with sli/cross fire.
I would also question the 2.4GHz cpu too. However as discussed before, overclocking it will bring it up to speed (pun intended), lol.
You have a nice system for sure. But the system it takes to run fsx full on won't be out until around late 2010 or so. That really is close to the truth, you know.
Anyway, most other games will probably run well enough on it. FSX is just one that (like the other FS's) is ahead of hardware available. There will come a day when it will run snappy without the need of any tweaks, but seriously, that day has yet to come for 'normal' pc's.
I hope you are able to get the 737 installed. Sorry I have no suggestion there, but I feel sure someone here can offer more help. Hoefully the tool Dave posted a link to will be all you need.