Agree with wildrock.
I'm sick of running a buzzy mini -> RCA cable to my receiver.
what's a "buzzy" mini? Even the G4 mini, with a good firewire or usb audio interface, then digital out to an AVR--makes a wonderful iTunes lossless system. If you currently have a mini, and your buzzing isn't a ground loop problem, stick a cheap M-audio in there, go coax or optical into your AVR and start enjoying your lossless music now with what you have. (If you have a wireless network, another inexpensive option for you is to stick an Airport Express right there with your G4 mini--connect it via ethernet to your mini in WDS mode--and then use its optical minijack out for lossless iTunes playback. Works quite well, I did that for a year or two in a bedroom before I upgraded. And don't be so wary of wireless audio until you've tried the Express, Apple lossless to an Airport Express wirelessly and out the optical minijack is pretty impressive from an audiophile perspective.)
AC-3 will always be more a function of the software, the player, you'll have no trouble passing it through from the Mac hardware perspective. Even the G4 mini could pass AC-3 as long as you routed it through a M-audio.
You also didn't mention which HDTV you have, that'll be the real determinant anyway as far as how smooth this whole "connecting to my HDTV" process will go for you, and whether you can get 1:1 pixel mapping, what kind of hassles you'll have with overscan, etc.
As far as playing back the illegally downloaded video content that might have AC-3 embedded in it, I think you'll find that most folks just use VLC--it's quite a versatile player for that sort of bt stuff, and yes, it passes AC-3 if the codec allows it. (Apple, Quicktime Player and AC-3 isn't the smoothest of routes to rely on.)
As wildrock hinted at, the Blu-Ray questions are premature, but the first Mac to get the capability will undoubtedly be the Mac Pro, so you should be fine there if and when that ever rolls out.
Mac Pros are very enviable.