Plan on new build solely as a media centre

Unsure if best place to post this.
I may or not, finance dependant, build a pc/media centre to locate by the tv. Obviously this will allow me to stream and watch on tv through hdmi. At the moment subscribed to love film, and am looking at setting up media portal and xbmc.
Now i have an old case or i could buy a media centre case, probably prefer the old case then less restricted on mobo and expansion slots etc. What the minimum i would need processor/mobo ram wise for it to function without problems. Would i need a graphics card esp to view at full HD(1920*1080 as in uk) I would get a new 500 watt psu, prob get a blu ray drive. TV card?
To save money i would put ubuntu/linux as OS, though would prefer win 7.
I have read about noise off fans etc, but surely the tv would drone out any noise.
I had a look at xbmc etc and couldnt get my head round it.
Done plenty of builds, just outdated.
Any info appreciated

You can use the old case as long as the video card will fit in it. The benefit of the MC case is it's small and designed to fit in with the other AV equipment in a cabinet or on the shelves. But if you have the room the there is no reason not to.

A pure Home Theater PC (HTPC) does not require a lot of computing power. So you can get away with a lower end processor and a minimal amount of RAM - standard speed too. The only caveat to this is if you plan on using a TV card that does not have hardware encoding. But most do.

There is no special power needs for a HTPC. The TV cards don't use a lot. You just need to size the supply to meet the needs of the video card which uses the most power in almost any system.

The fans and noise is a concern. In the scene where the little girl is hiding from the monster and holding her breath as it sneaks by is where you WILL hear the fans and it WILL annoy you! So you are looking for a silent build. HTPC enthusiasts used to covet fanless video cards for this reason. But it is almost impossible to find a modern fanless card today.

I believe Hauppuage still is the leader in high end TV tuner cards. I like them anyway.

The old case has just been dispatched to the tip, damaged. Thanks for the replies, at the moment i dont need extreme power which i knew but needed confirmation.