I understand that display connected to ADC via Belkin cable is not detected? Or detected with no video? Second one is working OK (via built in DVI)?
Reason may be: corrupted card's ADC output, incompatible display (Belkin cable is DVI-D - digital signal only).
http://www.datapro.net/techinfo/dvi_info.html
Try to zap PRAM - it will reset video settings to default (or reset nvram with Open Firmware:
http://www.regularjen.com/geeky-girlie/reset-nvram/) with both displays connected.
I used many G4 with dual displays with no problems (always auto detected) but i always one was ADC, i got no experience with Belkin or DrBott connectors.