Hmmm, that is quite odd indeed as that card gets decent benchies, not great but playable for sure.
Just curious, is AA enabled in your Nvidia control panel? Or perhaps the transparency AA (multisampling or supersampling)? That can be a killer too.
Ya know I got Crysis to run kickass in DX9 on the ole 8800gt, but as soon as I got close to the frozen ship it slideshowed due to the mist\smoke\vapors etc...
You may just have to turn something down a tad or buy another card, that's what I did