I've heard the rumors that ATV will play 1080p and it's been confirmed that the chip can and does support it but in all honesty, i really could care less. The only 1080p content out there is BD and BD content stored on HDD is IMO a waste of space, difficult to stream with all but the best networks and simply never looks as nice as it does on a BD player so.
As to audio out of the airport, it's bit perfect 48khz via mini-optical. My ATV also reports 48khz playback on the screen but others say it just aint so. I can't discern any difference and my 2 channel system is pretty impressive so again.....?
As for what receiver, that's really a budget call on your end and depends on the features you need. Mine are hooked to a Sony ES, an HK 347, a Pioneer Elite 21 and a Pioneer Elite SC-25 via optical or HDMI and all sound great.
If you can still find em and you're looking for a great deal, the Yamaha RX-V1900 was selling for $595 at electronics expo. It's a true 130wpc heavy weight that retailed for $1300 a year or so ago and a real steal at that price.