From the looks of it your hardware is fine to run this game. People who instantly tell you your hardware is no good have no clue. I ran Crysis on a similar system when it first came out just fine.
It would be helpful to know what driver versions you are using for your Nvidia card and what resolution. You should also be using SP2 for Vista as there are numerous fixes for the graphics subsystems etc. I remember on vanilla Vista x64 Sli was broken by the OS for well over a month before both MS and Nvida had to come out with patches to make things mesh...
With Nvidia cards sometimes turning all settings to low will cause a performance decrease. I can't explain the mechanics of it but I have seen it in several games when I had my 8800GTX setup. The biggest performance hit for Nvidia based cards now is the shadows/shaders. The 2 settings that you should focus on are AA and shadows. They will be the biggest hit against your performance.
Shaders, shadows, and particles seem to be the largest hits for pretty much all cards.
The mechanics behind the performance decrease is that certain features when on low have to be passed through a software based filter to compress textures, etc. Thus taking some of the load off of the GPU/vRam and placing more on system CPU and memory.
This would increase performance of a low end graphics card assuming the CPU and memory are able to handle the requests.
The reverse happens if the graphics card can handle the original textures as fast or at the same speed it takes to compress/optimize the textures via a software filter.
I've spoke with him, and he has literally tried everything except swapping out the CPU/memory/motherboard.
From drivers, game and GPU settings, to BIOS and Windows updates. Even swapping graphics cards.