Buy this:
http://www.amazon.com/Wii-Converter-Output-Supports-Display-Nintendo/dp/B005EK3OF4 to convert your Wii output to HDMI.
Use HDMI for your XBox.
If you need extra HDMI ports, use this:
http://www.amazon.com/gp/product/B0015YWKYY/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1.
I've purchased both and they work great and just take up a single HDMI port in my receiver.
If you don't want to go with HDMI, then the fat XBox cable should give you everything you need to pass to the receiver, unless you want digital sound, in which case you can use the optical cable. You will likely have to configure the receiver settings to link up these A/V sources. You also will likely have to have component cables from the receiver to the TV, because many receivers do not convert component to HDMI. The Wii may be able to have the video go over the component cable as well - depends which cables it came with. If you go this route, you will have to switch inputs on the TV to see the video. If you go the full HDMI route, you won't.