Recently I switched the main machine from an 8500/120 to a G3/266. In doing so I transferred the hard drive and PCI cards over to the newer Mac. Those cards were: a SIIG USB 1.1, an ATI XCLAIM 3D video and an Apple 10/100 Ethernet card. My monitor, a Portrait Display Pivot, had been previously attached to the ATI card while residing in the 8500 rather than the Mac's own video port. However, when I attempted to duplicate that arrangement with the G3 the screen displayed a partial flickering white image that contained nothing recognisable. Reconnecting the monitor to the video port returned a normal display, but this has me concerned.
These cards worked fine before, so there isn't an obvious reason for their misbehaviour now. I can only guess two possibilities: 1. The G3 may require PCI cards to exist in a certain order, although I haven't any evidence to back up that guess. 2. There may be a problem with the fact that both video sources, on the logic board and on the card, are ATI. I'm leaning in that direction, but don't know how to remedy the situation, as I'm hoping to connect a second monitor to the card.
Theoretically, the PCI slots are all equal, but sometimes swapping the cards around can fix problems.
I don't believe there is a conflict with the on board ATI video; many of us use other ATI cards (Rage128, Radeon, etc.) without a problem. You might do a PRAM reset; your video card's setting might have gotten screwed up.