Oops, don't forget the Flight Simulator X wikipedia page! This one shows exactly which planes you get and the difference between the standard and deluxe versions.
As to performance FSX is CPU bound provided you have fast GPU, so your FSX performance hinges on CPU speed. I take all my images with my normal in game settings, so what you see from me is running on the system in my sig.
If you have specific questions about add ons go ahead and post, otherwise every add on I own has been discussed in this this thread, which is true for many others as well.
Also take a look at Microsofts FSInsider site. You can get the FSX demo from here, so if you don't have it you better grab it.
EDIT: By the way most of the ground scenery you see posted is a product called FTX. Its a $30 add on and covers only australia at the moment. It looks so good many people have not flown outside of Oz since its release.
As far as the default scenery goes it covers the entire world with accurate elevation data, water data, landclass data, etc. The default is a lot more generic though than the high detailed add on scenery. FSX simulates the entire globe, as it is, so its pretty cool, especially with some add ons.