joetag,
I don't have a Wii myself, but I am pretty sure it ships with a composite video/audio cable - the yellow/red/white RCA connector cable. That should hook up in any of your available video inputs in the back of your Yamaha (looks like you have 5 to choose from).
For better video, you can get a component video - which has the red/white audio RCA audio cable connections and the Red/Green/Blue video connectors for video. Nintendo makes one, I've seen it on Amazon, and I am sure there are others out there. The RX-V1800 has three component video inputs on the back. Ahough the Wii doesn't display in high def, the component video cable will get you 480p video which is a better resolution than 480i on your HDTV.
I don't think there is an optical out for audio on the Wii (perhaps someone else can chime in here), so you are pretty much limited to standard stereo.
Mark
Aperion Audio Guru