Hi BOAC,
X Plane 10 is coming out soon and from the screenshots and videos that are available, it looks like it will be
THE flight sim to buy in terms of making things as realistic as they can possibly be within limitations.
More specific to your question however, I don't think their default aircraft will include FMC's and they're are not many payware developers for X-Plane.
The only think I can suggest is looking at this site, probably much, much better than FS2004, FSX or X-Plane but obviously much less rich in graphics, but then again if your not concerned about that then it is worth a look.
Welcome to Aerowinx!