The 805 is superior to the 508. If you need more HDMI inputs, check out monoprice.com and pick up some HDMI switches, and you can put 2 - 4 HDMI sources in a switch using 1 HDMI input on the 805. Also, if you don't think the 805 has enough power for your large room (which it most likely does, and if not, the 508 definitely does not), look into getting a standalone amp and using the pre-outs from your 805.
FWIW, I've used/owned two 805's. One I still use, in my 5.1 family room setup, although I am using a Sunfire Cinema Grand amp with it. The other 805 is in storage since it's been replaced by separates, an Onkyo Pro 5507 pre/pro and Sunfire Signature Seven amp. However, I won't sell my 805 as it is a more-than-adequate backup should anything else in my current systems fail me.
I still think, heat issues aside, the Onkyo 805 is one of the best receivers made from a price-for-performance and quality-components standpoint.
Two things are infinite: the universe and human stupidity. And I'm not sure about the universe. -- Albert Einstein