Well I use a Quadro2 Pro in my K6-2+ 600 MHz system and the card is complete overkill. As your card has about twice the fillrate and even more GPU power it will still be complete overkill.
For Unreal engine based games the Voodoo2 is a good choice and it will be faster than any Direct3D card. The performance gain from a second Voodoo2 is very low, however 1024x768 resolution can be a point to consider.

As a DX7 card with TnL a GF2 MX400 reaches already peak performance on such a K6-2 / K6-3 system.
Another way would be to look for a Voodoo4.

It is overkill hehe but I like overkill up to a point. I wouldn't waste a GeForce4 card on this system unless it was all I had.

The second Voodoo2 will be used for 1024x768 or stored as a backup. I plan to have all parts duplicated as backups to put into long term storage after the machine is complete.

I know everyone sees things differently but the way I see my time machine is it should be 100% DirectX 7.0 compatible. It will handle almost everything, reasonably well, from DirectX 7.0 and below, including DOS. Then I plan on a Windows XP box that will handle DirectX 9.0 down to DirectX 7.0 with overlap of DirectX 7.0 for demanding games that run fine in XP. This is just a personal preference. I am starting to look at a platform to build my new XP machine on. My plan is to end up having only two computers that are regularly used to play old games on and keep the others in storage or probably sell a few of them.