Almost all of nVidia nForce series chipsets are based on single-chip design (northbridge/southbridge combo).
Asus P5N-MX has nForce 610i which is also based on single-chip design.
So the northbridge you mentioned is also the southbridge; it's on the same chip.
The "video chip" you want to find is already integrated into the northbridge.
More information about nForce 630i/610i can be found here