Since apparently you have a 32-bit system I can recommend trying the 3gb switch described HERE.
Also, what's the amount and type of RAM that you have, which OS are you using and which version of nVidia drivers? Is it possible that either your CPU or GPU are overclocked? If so, it could be a good idea to check if it helps to clock them back to reference specs.
Checking if your GPU does not overheat is also recommended.
Another thing you should do is play on lower resolution, like eg 1440x900 (and possibly in a window), as your 9800 GT is equal to our minimum for nVidia - 8800 GT and these gfx cards will have problems running the game on 1920-wide resolutions. If you'd like to increase your fps I'd also suggest disabling SSAO.
Also, what's the amount and type of RAM that you have, which OS are you using and which version of nVidia drivers? Is it possible that either your CPU or GPU are overclocked? If so, it could be a good idea to check if it helps to clock them back to reference specs.
Checking if your GPU does not overheat is also recommended.
Another thing you should do is play on lower resolution, like eg 1440x900 (and possibly in a window), as your 9800 GT is equal to our minimum for nVidia - 8800 GT and these gfx cards will have problems running the game on 1920-wide resolutions. If you'd like to increase your fps I'd also suggest disabling SSAO.