You will need a 'humphy' machine for FSX....but once you get used to it, it is way better in effects than FS2004. If you go for FSX, check whether it contains the Acceleration pack. This is an extra expense otherwise.
FS2004 is very popular, and once FSX was sneered at even by me, but honestly when you get to grips with FSX there is no going back.
As an example of what can be done in FSX....we are carrier landing with arrestor wires, Catapault launching, Hoisting people out of the water with helicopters (if you can find them)..lol
Virtual cockpits are stunning now.
Some of the aircraft are so detailed now with total cold starts, you need to do every thing. I have the PMDG Jetstream, and all I have done so far is blow up ten engines and am still sat in the Hangar.
Some piccy's
This is me in the front Spit, with two of my online buddies in formation.
This is me flying the VC10, with a Tonka coming in to refuel back in my Rafv days