Hello Sir
I have a BV7 MK3 also and i must admit ATV was on of the best purchases i have made.
You can partially control ATV using the beo4 if your BV7 has Apple remote in the PUC list.
i dont have any knowledge about Airport express but as far as i know its so you can access ATV in link room i believe and you can do this using masterlink.
Sound loss i dont believe you woudl notice but it is all down to the bit rate of your music, i dont know what bit rate the 3200 rips your music at but if its good quality you shoudl not notice but im sure some sound tech will shoot me down here.
ATV is wireless and works in the same way your iphone works it just Sync all your music playlists etc from Itunes to the ATV.
a cool funtion of the iphone/ipod touch is the remote application so you can browse your music on the iphone/touch and play it through your Apple TV.
If you set up remote app you can use it select music from anywhere in your house and this helps with selecting music when in other linked rooms.
Hope i have been helpful i have only had mine for a month and i love it its great much better than the Beomedia and music more cost effective than the beosound 5.
Cover art makes a big difference in selecting music.
if you already have itunes set up then you are already half way there.
Beovision 7 MKIV (Blu Ray)
Beolab 9
Beolab 6000
Beo 4
Beocenter 9300
Apple TV
SKY HD
Optoma HD65 Projector
Lintronic TT455-RT-238
Beovision 3 MKII