In the game's "Video Settings".
1) Set the "Maximum Texture Size" to 256, or less. DO NOT SET IT ANY HIGHER THAN 256!
2) Do not use an anti-aliasing (slider all the way to the left).
In the game, under "Options->Graphic Detail"
The ONLY options you should have checked are "Bump Mapping" and "Detailed Water". All other options should NOT be checked.
I am making some assumptions here, so let me know if you are already running the above configuration.