Just install the latest version which is available.
See this page: FalconFly - Microsoft DirectX. Install the 'DirectX 9.0c' which is compatible with Microsoft Windows '98. (See the green checks).

To increase performance: Check every driver. That includes drivers for audio, chipset, graphics and so on. For audio/chipset just install the latest drivers. Regarding the 3dfx card: Try the latest drivers first. If that doesn't improve performance it's wise to try different or older drivers. Afterwards check the performance again and take the best driver.

The frame rate for GPL is capped by the program itself at 36 fps, so if your Voodoo5 is showing a steady 36 fps, then you have achieved the maximum possible frame rate.  Unless you have some sort of performance issue (i.e. stuttering problem in audio or video, etc.), there is no point in making any sort of adjustments.  The game's documentation should have information on the 36 fps cap somewhere.