The only reason to use DX9c is compatibility, but it is very slow. DX8 is not recommended even when it came out. DX7a is faster, it is the ideal choice if the game is compatible.
Win95 in general faster (+/-15%) and occupies less, but if your machine is powerful enough, with w98 you have everything, plus you have less errors.
I have not even tried to install Windows 2000, it is 64bits. XP is not possible. I think the current ceiling is WinMe (w9x).
If the game is compatible with XP/2000, it doesn’t run on Me/98/95… It has to be compatible, there are cracks, but it depends on the game.
Yes, it is 32-bit compatible, but keep in mind that you have two video cards. The voodoo is 3D only. And the S3Trio64 is all 2D, it is quite limited and suffers with 32-bit or high resolutions.
Anyway, you don’t need 32-bit to play, almost every game takes care of changing the graphic mode and resolution every time it starts.
If you want the highest speed, 256 Colors (even 16) and 640x480 is best, the more you have, the slower the emulator. But if you want to see images, manual or multimedia, of which sometimes games include, 16-bit covers it, you rarely found 32-bit images. Those were other times.
To cover everything (w9x) and the highest speed, it would be something like this:
Win98SE + DX7a, for everything compatible with DirectX 1 to 7.
Win98SE + DX9c, for everything compatible with 8 and 9.
and WinMe + DX9c, for the games that are only compatible with Me.