Aircraft can be modelled for FS2004 using the "Gmax" software, freely downloadable from
Discreet and the plugin should beavailable on the FS CD-roms.
For a more "realistic" or technical flight model, try
X-Plane from Laminar Research. The flight model in X-Plane is computed based on the actual 3D model of the aircraft, rather than generic parameters as Flight Sim uses, and has been certified as a training aid in certain configurations (I understand such "certification" is not much use to you, but it may be a factor in showing that the X-Plane flight model is more accurately computed).
As for the next release date, FS seems to be renamed every two years; FS98, 2000, 2002, 2004 etc; and released a year earlier, so unless MS bucks the trend, FS 2006 should be out in 2005
God I'm sad, real flying is more fun! 