Why the am I experincing lag to an unplayable level even on lowest settings?
I don't know why you are but I have a Quad Core 6600 and the same awesome graphics card but only have 4 gigs of DDR2 RAM, Windows 7 Ultimate and I get acceptable and smooth frame rates. My only issue is graphical anomallys with the textures when you swing the camera view around or inside the Algonquin- Alderney tunnel, that lasts for a couple of seconds.
I had to get my motherboard replaced and at the same time decided to dump Nvidia altogether. I switched to another Asus mobo with the new Intel chipset and replaced my two 9800 GTs (512 mb ea.) with a single HD 5850 (1GB). I didn't notice the game running any better or worse after the hardware changes. I still notice disappearing and other texture problems in the game. These only occur for a second or two when you change the camera view but it's noticeable and annoying. Performance wise the game runs smooth and frame rates never dip at all with games resolution maxed to my monitor. Make sure ATI users especially, that you get the latest Catalyst drivers.
I read that if you can use Riva Tuner there is a way to fix texture/graphic issues in the game but the steps can appear complicated if you're not familiar with the program which I am not. If anyone knows how to use this program and apply the "hot-fix" for this game pls. post here.