I'll contribute the little that I know
If you use a cable box to get channels (as everyone soon will for HD), that OUT signal can be split to go to a capture card/tuner and a TV (what I do). If you have no need to split the signal, you probably wind up with better video by sending the signal straight to a monitor/TV. You can use a single device as both a TV and a monitor, with a control switch for the monitor determining how the device is used moment by moment.
So you have options.
Cable path equals LINE to Box...the output from the box has several options, as noted.
If you decide to use a splitter, then you have decisions to make. I can split my basic cable (the majority of channels I watch) at the Cable and ignore the channels I need the box for...or I can split the output of the box and reflect the same channels as my TV. When I split the output from the box, I am merely using my computer hard drive as a VCR and have to tune the capture card to channel 3 to reflect channels which my analog capture card don't allow me direct reception.
I don't use On Demand, but it seems to need the Comcast box to exercise.
And...if you use a splitter, I suggest making sure that you use a reinforced cable for your Comcast box out. Generally speaking, it has been suggested to use one of those amplifiers just before the splitter (which I have done before), but I believe that's not necessary if the proper cable is used to minimize leaking TV signals.