Set the in-game "Maximum Texture Size" to 256. Check the option to preload textures into system RAM.
Check both the "optimize" options.
Now to the fun stuff.
A video card is only as fast as the data can be gotten to it. If the CPU is too busy, frame rates will suffer.
Make sure all programs are shut down, then from the Windows desktop, open the Windows Task Manager (Ctrl-Alt-Del). Select the "Processes" tab.
For Windows Xp, there should be around 19 to 21 processes listed. Look in the lower left corner for the total count.
Next, check the CPU usage immediately to the right of the total count. It should be bouncing, at regualr intervals, between 0 and 1 or 2 percent, and no higher. Watch it for a minute or two.