I recently acquired Apple TV, but all in all I'm not that convinced by it. The menu's are shocking and it's bloody slow. The worst thing, and it's truly awful, is that you have to have iTunes running on your PC (permanently open as an application, not exiled into the system tray, or background service) for it to share the content with the Apple TV box. Another annoyance is iTunes insistence on only interacting with MP4 format, which means I'm having to re-rip my library.
The only thing it's got going for it is the Airplay (so you can instantly get a slideshow from photos on your iPhone/iPad onto the telly). I would say that I'm attempting to run the Apple TV through a WiFi connection - but I've already priced up getting a wired connection to it because I'm hopeful that it will speed things up a bit.
I'm glad someone mentioned RaspberryPi not decoding 5.1 as I'll make sure to rip to stereo. At the moment I have an original Xbox 1 with XBMC running in one room and was looking at a Raspberry media centre to replace it - although I'm still quietly impressed the old xbox still runs!
If you've got a PS3 already (I wouldn't bother with Wii - I assume you're running an HD telly) and a PC, and both are connected to the same network (Wi-Fi or wired) then you should be able to simply enable sharing in Windows Media Player. In Windows Media Player (WMP) look at the library section and select 'Stream'. One of the options is to "Allow devices to play my media". Enable that and your library should be visible to your PS3. Assuming that you have some media in your library of a suitable file type you should be able to stream this directly to your PS3.
Typically, here's a youtube vid of a kid to tell you how to do it:
http://www.youtube.com/watch?v=psNFEyeRiL0It is one of those things that everyone has a different opinion on though, and I wouldn't vouch for WMP being 'secure', but that's probably more down to Network security I'd imagine.
Good luck getting set up
