Your decision may depend on the rest of your set-up.
The G4s ship with an ATI card in them already, and you can upgrade this card to a Raedon, as you know. In either case the cards will come with a regular VGA connector and the new Apple ADC connector on them. If you are planning on buying Apple's new studio display monitors then you will HAVE to have one of these two cards in your G4 because the monitor has ONLY an ADC connector. There is currently no way to hook up the new Apple monitors to a VooDoo card, or any other kind of video card.
In addition, if you buy the Raedon separately (ie, not in your build-to-order configuration) it will NOT have the ADC connector on it from what I am told.
If you have 2 monitors then you can do whatever you want. I have the stock ATI card with the new Apple display hooked up to it and the VooDoo 5 card with an older monitor hooked up to it. Everything works great.
Also, 3DFX recently announced they are coming out with an AGP card for the Mac. This is good news, for the most part, but I'm sure will leave a lot of people pulling their hair out. If you buy the AGP Voodoo card you will have to remove the ATI card. Then in order to hook up a second monitor you will need to buy ANOTHER video card. And if you only have 1 monitor and it's the new Apple monitor then when you install the AGP Voodoo card you'll be out of luck because you can't hook up the Apple display to the Voodoo cards. There is an ADC to VGA adaptor, but from what I understand it is for hooking up regular monitors to the ADC connector on the card- not for hooking up Apple monitors to regular VGA connections.
So basically, you need to know how many monitors you will have and what kind. That could make the decision for you.
->Later.....Spice