There are some topics talking about it, but to be honnest, just like you I've never been so lucky with the "Search" tool of this forum

FSX had two versions: normal and Deluxe.
Deluxe had two more planes, Maule and Goose, some more detailled airports, some more missions, and the SDK for those who are able to build sceneries or planes or any other kind of addons.
Then came Acceleration. Acceleration is an addon for FSX, with three more planes (Mustang, Merlin and Hornet), more sceneries, more missions, and some more features (aircraft-carrier operations, engine damage, Reno-style air race, etc...).
Finally came FSX Gold. That's not a new version, in fact it's just a pack, containing FSX Deluxe and Acceleration, so you really get everything.
Important thing is that FSX Gold is less expensive than buying FSX Deluxe and Acceleration separately. So FSX Gold would be the best choice.
Now, when it comes to your computer, you have to be warned. Contrary to FS9, FSX will NOT work very well out of the box. You will have to learn the "tweaks" to make it use efficiently the power of your computer.
Tweaks are just modifications of the FSX.cfg file. Perhaps you already did something like that with your FS9.cfg file ? There are parameters that you can modify, and parameters that are hidden, so you have to add them and set them to correct values.
There are plenty of informations about the tweaks, just take your time to read the sticky topics and the replies from NickN. It will take a while but in the end there's nothing too complicated.
FSX will be more frustrating than FS9 to be set up correctly. However it will bring you different things to appreciate, things that you won't find in FS9, things that depends on your feelings, things that you may appreciate or not. It's up to you. If you have some money to spend in a game, it's worth a try. It won't damage your FS9 anyway
