I'm using a 22" cinema display and a 17" Apple flat panel, via the Dr. Bott DVIator, all with a dual 533. With sessions that have a lot of plugs and tracks, I often get the PCI bus too busy message. This happens often when I'm doing fader moves while a track is playing. I hope someday that future hardware and software will manage to fix this, because the cinema display is fantastic, and having the mixer on 17" doesn't hurt either.
I don't use the USB ports on the 17" to try and minimize power consumption. It's weird though because in the same session, sometimes the PCI system usage window will be in the red, and sometimes in the middle. Part of the problem I think is that I have an Ultratek raid card too. I use two 40 gig drives for video capture and edit, and ProTools will never let me record to those drives - I guess because of the power needed to run the card, the 001, and the DVIator. What I'm not sure about though, is how much power is being usurped only by the card's presence in the bus, even though it's idle.
Sean
www.seanmalone.net