Have a look at
this. The RGBI port is a digital one, which will only work with a CGA monitor, or one of the 1084 and compatibles.
However, as the RGBI output is driven by the VDC-chip, which can be set to output just about anything, some monitors may have trouble displaying certain screenmodes. If your C128 has 64KB VDC memory, you can go up to some 720x700 pixels (overscaned 640x400 interlaced)...
