It will probably use more power than a Mvix/Popcorn hour but not much. But it can do so much more. It's just a normal pc with TV as a screen. I've got an old Antec HTPC case with an Antec 240w PSU. It will also depend on how many juice sucking components you have. I dont need an optical drive so it basically just have to power the harddrives and motherboard with onboard display (couple of watts). Oh, and a wifi pci card. I also have a WD 500gig, Samsung 1tb and a WD 640gb external connected. I'm using a videomate K100 remote control, was about R200. The rest of the specs is in my sig.
I'm currently running Vista Media Centre with MediaBrowser. Everything looks beauitiful with dvd covers, hi def back drops, actors, movie/series descriptions, first air dates and everything automatically updates with no user input required. Just copy your movies/series over in their seperate folders and mediabrowser does the rest by downloading all meta data automatically. There are also loads of other plugins like weather forecasts/browsers/internet tv/radio/games etc. Heck, I could even play Guitar Hero 3 off the onboard ATI 3200HD. Video is connected to my plasma via HDMI and sound via optical to a Samsung amp. My next upgrade is a decent digital tuner to turn it in to fully fledged PVR.