They will work together. I don't know if there's any particular order to
GEX and UTX will fit together like a hand and glove.
There should be no problem adding Flight Environment X.
I have Ground Environment X, Ultimate Terrain X and Real Environment X.
Those 3 have transformed FSX into a completely different world.
There's very little, if any, frame rate hit with those three.
Intel Core i7 975 Extreme (3.86GHz)
12GB Corsair Memory
Nvidia GeForce GTX 285
Windows 7 Home Premium 64 Bit