it could be a bad connection in a cable somewhere, or possibly in a monitor. Some motherboards/video cards are more sensitive to shorts and miss communication than others. This could just be one of the freak combinations with the used hardware.
The other thing you could try, as i've seen this work, use the monitor/video card combo which works and uninstall all drivers for all video cards and monitors, including software which may hold values relating the 2.
Then shutdown your comp (without detecting new hardware) and install the desired parts and boot. Without any drivers or software possibly telling the boot.ini file and bios what to expect, it might just work
