Tarik,
i would definately recommend FS navigator like Jose - not much use with pmdg or level-d planes that have FMCs, but great with genral aviation planes and those that don't have FMCs such as the beechcraft 1900D.
I also use flight environment which gives great clouds, water effects etc
Both do effect frame rates a bit but not terribly - depends how good your system is i guess??
jim