its a massive step up in terms of audio quality, power handling, functionality ect. i just cant recommend you buying one if all you want is hd audio support. when i bough mine i bought it because it did more of everything (over my yamaha rx-v630) and did it all so much better. more connectivity, hdmi switching, vastly better audio capabilities and processing (THX of course), capable of biamping the front speakers.
it's just in a different league and hdmi is great. its in a different league to the 605 as well. if you want to know why the 805 is worth of its price tag over the 605, start with the weight of the unit. its twice as heavy at 23.3kg. thats a good indication right there.
however, again, if you are upgrading for audio and nothing else, i just cant recommend buying one. its a real waste buying a unit that can do so much if you are only interested in the audio. go and get a demo of a few amps if you can, that'll help you decided on what to get. and ask on av forums about 2nd hand avamps as well. they may well find a few bargains for you.