What video card do I need to watch 1080P MKV files?
If all you want is just decodeing + vector adaptive deinterlaceing, a 5450 is enough.
However the 5450 isnt powerfull enough "compute" wise to be able to handle lots of post processing effects as well. The 5570 is, which is why Anandtech named it the HTPC card of 2011.
So if you want to tinkere with video settings and do some postprocessing effects get the 5570.
HIS has a passive cooled version (for around ~59$):
http://www.newegg.com/Product/Produc...82E16814161380
Remember to do some undervolting on your cpu/ram, get system stable,
running with less volts (overall power useage).