You want to capture video on a Pentium Pro machine? I dunno about that idea. Disk I/O performance and CPU performance will be a problem. Some encode software really likes MMX and a PPro doesn't have it. Hardware encode acceleration will probably be necessary.

A PCI Radeon of any sort will provide good hardware MPEG 1 and 2 decode acceleration.

Matrox had a daughtercard called Rainbow Runner that did MJPEG encode, I think.

It is more for technical demo purposes or maxing out the system rather than for practical video editing. I may go with a PCI Radeon AIW of some sort then. It would also be cool if I could switch back and forth between the Voodoo and the AIW, I think Window 98 supported multiple graphics cards but it might require a reboot, not sure you could switch on the fly like that.

Each card will drive own display, so you can just switch output.

It is possible that a PCI Radeon won't work in your board. (I experienced that with a Radeon 7500 PCI on a Asus P55T2P4.)
ATI included full MPEG2 acceleration from Rage 128 so in case you run into problems an All-in-Wonder 128 PCI might be a solution.

What about a Geforce2 MX ?

This from THG might be useful, I don't know how many of these cards are available for PCI though:
http://www.tomshardware.com/reviews/multitalented,144.html

Also no idea how a Pentium Pro would fare with one of those cards and the lack of MMX ++ being older than most of those cards/GPUS/etc by a reasonable margin. Pentium Pro came out in 1995; that review is dated 1999. What you want may or may not work out very well at the end of the day - as swaaye suggests.