FS2004 is the one I use. I still have the FS2000 CD's somewhere but uninstalled the game right before I installed FS2004.
There is only one chance I'd buy FSX - if
-it was a cheap buy,
-there will be no successor
-I finally have the hardware to run it the way FS2004 runs right now
If all these things would happen I might consider buying FSX

Also it took me many hours to have such a nice environment consisting of various sceneries (payware and freeware), enhancements and traffic. The FS folder size is 19GB (I recently removed some sceneries as I am limited in HDD space

)