It depends on which signal you are talking about: audio, video or both. But first let's get the terminology straight.
A dvd player or set-top box are source components. They are the source of the audio or video, so you never bring a signal IN to those components - they have outputs only. You can connect the component video outs of the dvd player to the component in of the receiver and then run component ('monitor out') from the receiver to the tv [or you could connect directly to the tv and not use the receiver]. Likewise, the audio out of the dvd player goes to an audio in of the receiver (whether it's optical or coaxial digital or analog audio). The same would go for the set-top box if you were to use one.
The cable card allows you to tune the cable channels. Just as if you were to only use the cable from the wall, the cable signal contains both the audio and the video. Presumably you want to get the audio from the TV (which is now acting as a source device, because it is the source of the cable signal) to the receiver. You would need to run a cable from the audio OUT of the tv to an input on the receiver - hopefully the tv you buy has a digital audio out so you can get DD to the receiver. You don't need to do anything for the video - the tv already has it.
The disadvantage of cable card at this point is that it is one-way. It cannot communicate back to the cable company head-end server. Therefore you cannot use any interactive services like Pay Per View or Video On Demand. You also can't get the cable company's program guide, although some cable companies will download something similar (like TV Guide) to the cable card.
