The 804 is THX Select II certified so it will have a lot more power than the receiver that came with the HTIB but that only really matters if you need that extra power because your room is large and/or you like to play things really loud. Of course it does also offer many more features like video transcoding, HDMI, and convenient settings for things like Max Volume, Turn-On Volume, video delay settings, and many surround mode options - including the THX post-processing.
If you don't care for the extra THX processing, the 674 is the current sweet spot of the line and offers many of the same features.