Well, to answer your first question, Lockheed Martin picked up the FSX platform in the form of what they call "Prepar3D." Here is a link ot their site: http://www.prepar3d.com/
I haven't heard of A2A developing a sim to replace FSX. They have their own stand alone sim in the form of the Battle of Brittain II and some talk about making a sim for the Flying Tigers but that's still just kind of a discussion.
In regard to your computer....I would focus on your memory, video card for a performance jump.
Hopefully this was some help to you.