These are alot of questions, I'll try to answer them step by step
If I wanted to try this again, would I have to turn off 'Fast Write' ?Fast Writes is an AGP-specific performance Option, therefor it will affect only your AGP Card (Radeon). It is known to cause severe problems on some Systems (e.g. many AthlonXP owners have to turn it off to ensure stability).
Technically, this will not affect your 3dfx PCI Card at all, so enable it if you can, disable if your Radeon/System becomes unstable .
I don't know how to disable 3d. I do know that the Voodoo 1 works with 2d cards, not 3d, unless there is some way to disable 3d?Actually the Voodoo can work just as fine with 3D Cards as well, but it will always (with the exception of Glide which the Radeon doesn't support) remain the secondary 3D Adapter. Thus, it would never be used unless specifically enabled from within the Game Options (if such an option is available, that is)
There are a couple of older Utilities to address this Problem (at least for Direct3D) floating around, but they haven't been updated in a long time and likely are not tested on modern Systems (should still work though).
Also, would I be getting the speed of a ATI 8500 or a ATI 9800 Pro? because I know the voodoo 1 is a video acelerator, but since you can't acelerate something faster it should run at 229Mhz or 380Mhz right? Also my current system has a 16x PCI slot. The 3dfx Card will not speed up your Radeon nor the way around. Both will still run unchanged at their specifications.
The term "accelerator" was born at times where there were only 2D Cards in use. As soon as the 3dfx Card takes over, it will 'accelerate' 3D, while the Primary Adapter is basically not used at all anymore.
You have to select which 3D Adapter to use and consequently will get the normal performance and features of that specific Adapter alone.