Uncompressed BD rips are anywhere from 12 to 30+ gigs per movie - its a lot of space but its worth it if you have a display that supports it and can swing the storage.
That Acer box thats linked will run XBMC, but the UI might run slow if you run a heavy skin (ie Aeon). Video playback if you run the linux version will be flaweless and smooth using gpu VDAPU, but the windoze version doesn't (yet) support gpu playback.
XBMC rocks - I just bought 2 Asrock 330 ION PC's to replace my aging HTPC's running DVDLobby / ZoomPlayer and it effing rocks.
Oh, and that Boxee box is effing akward! Um, why? I don't care how small it is, but um, you could try to make it "fit", no?