The mobo possibly has a builtin display controller - you can check in the BIOS. If so, you have to disable that, so the ATI card can use the I/O ports and memory addresses. It's even possible (with ATI hardware) that an onboard PCI display adapter could interfere with it. In any case, disable any onboard display adapters in BIOS.
Also, set all the PCI steering IRQs to 11 or something like that - try 11 and 9.