While it is possible to get 6 monitors hooked up to a G5 quad, It is entirely another thing to get all 6 monitors playing full frame video, frame accurate. This is exactly the sort of thing my studio does constantly, and I can tell you that a one machine scenario is no go. It's got less to do with the graphics cards than it does with the ability of the SATA (or other) drive bus to deliver the media without any hiccups. After endless experimenting with this situation, in a case where we wanted 6 outputs, driven by Mac, you're best served with one mac per two outputs. Even better is to add an additional for unimpeded control and monitoring. Speaking of which you can jigger the runtime environment in Quartz Composer, or you could also whip this up in MAX/MSP Jitter. Another option would be professional level multi-screen presentation software like Dataton's Watchout or AV Stumpfl's Wings Platinum.
The main thing here is to split the load between multiple machines. Like I said, this is mostly to do with drive bus and Front Side Bus MOBO bandwidth limitations.
MODE Studios http://www.modestudios.com
Contributing Editor, Entertainment Design Magazine
Art of the Edit Forum Leader
Live & Stage Event Forum Leader
HD Forum Leader
Build a massive comp in AfterEffects that places all your elements in the respective screen areas and hope somehow the video cards figure out how to show that...
or use one of the specialized machines used in live/stage to multiplex video over multiple screens. Guys in the COW live/stage forum would be the ones to answer that one, it's one of the main things they do.