Andrew,
Alright. The Onkyo has four digital audio inputs - two are coax inputs and two are optical inputs.
Your new DVD player can output both coax and optical, your PS3 can (from what I can find) only output optical, your Xbox can (from what I can find) only output optical, and your TV can only output optical.
So...here's what I would do for audio:
1. Connect the coax digital output from the DVD player to one of the coax digital inputs on the Onkyo receiver (the coax jacks use RCA-type connectors, and you could use any cable with an RCA connector on both ends)
2. Connect the TV optical digital output to one of the optical digital inputs on the Onkyo receiver
3. Connect either the PS3 or the Xbox 360 to the second optical digital input on the Onkyo receiver
For now, you should be able to listen to whichever game console you didn't connect with an optical cable to the Onkyo by listening to it through the TV's connection. Audio from that console should get passed by HDMI to the TV, and then the TV should output the audio through it's optical output back to the Onkyo.
If you want, you could get an adapter that converts a digital optical cable to a digital coax cable (like
this one at monoprice) so that you could connect your second game console to the Onkyo directly using the Onkyo's second coax input.
Does that make sense, or is it more confusing?
EDIT: Wasn't trying to override Phil - he and I were just typing at the same time.