Textures for FSX in .dds format will need flipped vertically. FSX supports .dds DXT1 format (CFS3-compatible) and DXT5 (not compatible): conversion may be needed. Watch the alpha channel too: FSX uses alpha to control reflection or opacity, depending on how the original model was built. Compiled models are not CFS3-compatible but gmax source files can be made compatible for anything gmax can export to, with some work. Sounds are fine - I have an FS9/FSX soundpack for my model which sounds terrific.
One consideration: CFS3 expects a single texture file (without alpha) for the model if the .mos (damage effects) file is to work: FSX models often use lots of texture files.