The short answer is that I'm fairly sure pairing the Voodoo 3 from 1999 with the Pentium II from 1998 would improve frame rates a bit. The Voodoo 3 was supposed to be about as powerful as a Voodoo 2 SLI setup, and the Pentium III didn't even exist when Voodoo 2 SLI was the best form of 3D acceleration money could buy.

However pairing a 1999 3D card with a 1998 CPU would still leave you CPU bottlenecked, because if I'm not misremembering articles from 1998 I read on Tom's Hardware, the CPUs from that year didn't fully tap out all of the Voodoo 2's 3D rendering power. And my 300 MHz Pentium II isn't even the fastest CPU you could get that year.

Nonetheless what you're proposing does sound interesting, and I'd like to try it later. The only hold-up is that my more pressing goal is to compare the 1998 PC's gaming performance with an ATI Rage 128 instead of a Voodoo Banshee, because I want to determine whether or not 3DFX was really the best choice that year.