I tried this out on my new PC as well and was shocked to find that I could run it with every single setting maxed out (DX9 full lighting included) with 4x AA and 8x AF and motion blur...and the PC holds a rock solid 60 fps. I honsetly didn't expect that after hearing how slow the game was running. I mean, it was smooth on my old PC using much lower settings (no dynamic lighting), but DX9 would drop the framerate to sub-10 fps previously.
The game really does look different with DX9, however. Without dynamic shadows and HDR, the game feels VERY dark and dreary (not necessarily a bad thing), but I never realized it was supposed to be sunset during the intro areas.