Video wise, the only thng HDMI has over component is that it handles 1080p. Also, some devices(upconverting DVD's, Hi-rez DVDs) will only output the end result in HDMI. My Comcast/Motorola HD cable box puts out a mighty fine 1080i picture.
As for sound, depending on what source you're using, you may or may not het anything except stereo. Check your manuals. both source and target and see how they handle HDMI audio.
but, you can always get MC DD/DTS sound via toslink or coax directly to your AVR, assuming the source device and software offer it.