I have noticed a weird occurance when i am playing the witcher 2. I am playing this game on max settings and getting 60 -70 fps, but after 30 minutes i notice that the fps lowers dramatically to 15 fps. I also notice that the cpu usage drops to half. Whats wrong with my video cards?
My specs are
i7 920
Nvidia geforce 460 756mb SLi
Any help would be appreicated

Also turn down the texture memory size.  Below is taken from Geforce.com's Witcher 2 tweak guide section for Texture Memory Size.
 
 
Texture Memory Size (MB)
 
This setting determines the amount of memory which the game will allocate for storage of textures. It has no real impact on FPS, nor does it alter image quality in the game. It only affects the amount of stuttering and visible texture loading which occurs as you wander the game world. The optimal setting to use depends in large part on the amount of Video RAM (VRAM) on your graphics card, as well as the Texture Downscaling setting covered above.
The available settings are shown below, along with the corresponding maximum amount the game will allocate to storing textures in memory:
 
Very Small = 100MB
Small = 200MB
Medium = 300MB
Large = 400MB
Very Large = 600MB
 
Remember that all of your graphics card's VRAM can't be used just for storing textures, so it's best not to select a setting which is greater than your actual VRAM amount - this may increase stuttering or hitching, and even cause crashes in some cases. At the same time, choosing a value which is too low will result in moments of noticeable texture streaming during the game. Start off with a value around half of your actual VRAM (e.g. select Small for a GPU with 512MB VRAM, Large for 1GB of VRAM, Very Large for 1.5GB of VRAM or more). Then if you don't run into any problems after a while, raise this setting to the next largest value and play further to see the impact.
 
For those with less VRAM, a combination of choosing a reasonable amount of memory to allocate to textures, and employing some Texture Downscaling to reduce the size of textures will result in the smoothest and most stable gameplay. You can also set a custom amount of memory to use for this setting as detailed in the Advanced Tweaking section of this guide.