From what I understand, back in the day you would need 2 graphics cards for games. One to handle 2d aspects and the other being 3d. Am I correct here? Obviously now just about all graphics cards come with both built in.

That's great- but to build a tower using only voodoo cards, what do I need to focus on if I decide to get something like a 3dfx voodoo2 card maybe as an SLI let's assume or any other voodoo card as long as it is PCI?

In a classic setup you would need a 2D card combined with a Voodoo Graphics (Voodoo 1) or Voodoo 2. The Banshee and Voodoo3 and newer cards have a 2D chip built-in so they don't need a regular 2D card. The Voodoo Rush has a 2D chip on-board.

To be precise; the Voodoo Graphics and Voodoo 2 don't really 'need' a 2D card to work. It's just that you need the 2D-card in order to run DOS/Windows or to run 2D games. Purely for 3D games it enough to only have a Voodoo add-on card.

PCI or AGP has nothing to do with 2D or 3D. You can choose either bus you'd like. AGP is a bit faster but it's a small difference compared to PCI. Especially with the slower cards.

Reason I said PCI was because I'm wondering what I should get (sorry for confusion)
since the tower I want has ATI RAGE IIC, only has PCI slots, and I figured I could just add a voodoo card in.

If I were to just add one, what would you recommend? Voodoo Rush? Voodoo3? Voodoo2? In case I ever want to play some 3d game.

Anyways how would anyone even begin to use a pc that doesn't have 2d capabilities? Or is that just left to some on-board graphics? I think I'm beginning to understand now.. :D

If you want to add a card (next to the existing ATi Rage IIc) then you should either choose the Voodoo 1 or Voodoo 2. Other cards (like Voodoo Rush and Voodoo3) already have 2D-functionality. The Voodoo 2 (and especially in SLI) is fastest but few older games might only run with the Voodoo 1. Nevertheless I would probably pick the card which is 'time-period correct'; i.e. a system from 1997 with a Voodoo 1 and a system from 1998/1999 with Voodoo 2.

If your PC allows you to boot without primary 2D/VGA card then you would be able to start up your PC and start a game by typing 'blind' or using a start-up script. Once the game loads up it will switch to 3D and produce an image.