Set to 24-bit.
My dad's Plasma HDTV supports 30-bit as well (Deep Color) however for it to work requires an Input device (i.e. BluRay player) and the Video to all to be in this mode.
I'm not sure there is any media content that supports this. Even if there was, I think you'd need a Plasma screen to really notice the difference in quality.
I'm not sure why you have the options to support more than 24-bit since your monitor Manual seems to indicate it doesn't support 30-bit or better. Perhaps the XBOX 360 can't really tell what your monitor supports?
*You can try 30bit if you like, but I suspect it either won't work or else video quality might look weird.
Another option is that certain PICTURES might be 30-bit or better, but again I didn't see an indication your monitor supports this. I could be wrong though.
**I think you'd have to contact BenQ to be certain as the Manual simply isn't clear on this.