It's actually not that intense on most machines, just don't expect to run it at high res.
Special Note: you can actually change the amount of VRAM it will use, and how big the texture cache is using the console. This leads to HUGE performance gain on all systems I've seen it used on (although my buddy with the x800 still hasn't gotten it in yet).
I've noticed that in some sections they don't use just shadows, they actually alpha out the geometry to make it black. Take THAT all you gamma strumpets!
I guess the mods didn't like my last comment, is this better?