The Onkyo can switch HDMI (I see the DV400 has HDMI output) but if your TV doesn't support HDMI then it's moot. Otherwise, you can run component video and optical/coaxial digital from the DVD player and from the cable box to the component and digital inputs on either receiver (assuming your TV has component video inputs, you can use S-video or composite on either as well if needed.) If you don't need HDMI, I'd lean toward the Yamaha myself, it's a very strong receiver. Hard to go wrong with either, though.
Are you just running the DVD player to the TV right now? What connections are you using for that (HDMI, component, S-video, or composite?)
TP