With the equipment you have I wouldn't worry too much about introducing HDMI but would recommend using your receiver to more advantage while at the same time reducing the amount of wire at the back of the TV. All audio and all video should be going into the receiver and then just video (one composite cable and one component cable) going out to the TV
Connect the Wii (video 1 or 2), the XBox (video 1 or 2) to the receiver via composite (RCA Red/White/Yellow) in then the receiver to the TV via composite video (RCA yellow).
Connect the Blu-ray (DVD In) and the cable box (video 3) to the component inputs of the receiver and then a component out to the TV.
The Wii only has component video last I checked but it is a nice upgrade. The XBox does have an HDMI option which would then add a wire or to to the back of your TV!