I understand the 9800gx2 requires a 6pin & 8pin connection from the psu, are both connected?
Does your version of the Asus M3A have a 4 or 8 pin atx12v connector?
The more you ask the card to do, the more juice it needs, a 4pin m/b atx12v connection probably can't supply enough to the pci-e slot, which the card also draws power from.