Not entirely accurate, to be honest with you.
On one hand, FSX is the same as FS2004. For example, those with low-end systems will experience FS9-like graphics because the current hardware won't be able to handle the high system requirements set by FSX. One very good example is my computer. My computer is so old, that it doesn't even support DX9; as a result, my computer can't render reflections or any feature that requires shader 2.0 support.
But on the other hand, FSX is completely different. If you look beyond the visuals and into the technical, you'll see that there is a huge difference. For one, FS9 can't render a "round" planet so it has to deal with a "cylindrical" planet. This imposes a huge limitation on how and where aircraft fly [can't reach the poles and can't fly higher than 99,999ft MSL]. Now look at FSX. In FSX, you have a "round" planet; thus allowing you to fly litterelly into space [up to 18,000 MILES] and you can finally reach the poles [this was one of the biggest complainst from users]. In FS9, you need 3rd party utilities to provide better viewing options [which not everyone is aware of]. In FSX, those additional viewing options are now stock. This can be very convenient for those who don't even know how to install those features if they're new to the sim. FS9 also fails to support inverse kenetics [IK or skin and bones] while FSX supports this. IK in FSX can be used for both scenery and aircraft design.
Also, FS9 doesn't support
DDS textures while FSX does. On top of that, FS9 can't support the new ultra-high terrain resolutions that FSX can. And one more thing, I noticed that the terrain in FSX is much more friendly with surface vehicles compared to FS9's terrain.
So, visually [assuming that every user has a low-end machine] FSX looks the same as FS2004; but techincally [if you look at the bigger picture] FSX is a whole new level.