Yes iTunes has HD streaming, it won't be nearly the quality of blu ray, but it is available. Netflix has HD streaming as well, so does amazon on demand. Depending on the quality of your internet connection, the HD streaming is usually better than DVD, but far short of blu-ray.
Use either an external thingy (apple tv, roku etc) or plug the A/V receiver directly into the router and stream through netflix or whatever, or depending on your tv, you may be able to connect that to the router and stream from there.
Personally I use my panasonic plasma and stream from amazon on demand since I'm an amazon prime member.
You can also hook up your computer to the tv or avr with an hdmi cable if your computer has an hdmi port and just do the whole external monitor thing.