Both cards can stay. Macs work very well with multiple monitors, and you will be able to run dual monitors from the two cards. When you install OS X, you may need to not have a monitor plugged into the V4. That is just during the install process, once OS X is up and running you should be able to shut down, plug the Voodoo's monitor back in, and boot into OS X again.
Note that the V4/V5 do not currently support DVD playback, you will need the ATi card anyway if you want to play a DVD movie.
I do not have the V4, my 2nd monitor is running off an old ixMicro card that I got on the cheap.