Found this thread... thought I'd comment. So I have a socket 754 3000, 1GB DDR400 and a 7900GT CO, I keep experiencing hitches with next generation content turned on. Game is patched to 1.2 and running the latest forceware. I see that the game recommends a 512 video card, so I guess I need to step up when the G80 is released. I really wanted to play this game with all the effects turned. My 7900 gt plays everything else I throw at it beautifully. IMO this game is badly coded.
I will try disabling the shadows when I get home, to see if that gets rids of the hitches. I hope Eidios releases a 1.3 patch or Nvidia releases a new driver that increases the performance.