It actually works on both FS9 and FSX right now on almost any version of XP 32 Bit or 64 Bit, English, Spanish, Swedish, Japanese, whatever...
What is still in development is support for Windows Vista 32 Bit and 64 Bit. But that should be done by early January 2009 at the latest.
What more, you don't have to decide whether you are going to need it for FSX or FS2004. Just install and run any sim you like.
Subject: Will the FS9 version work in FSX? 
