Leon Yiu

http://www.avsforum.com/forum/26-home-theater-computers/1477339-so-you-ve-built-your-htpc-now-what-next-how-get-ultimate-picture-sound-quality-your-htpc-madvr-svp-xbmc-mediabrowser-jriver.html
 
For those worried about the effects of scaling when watching less than 2160p footage
 
If you use windows there is a high quality video renderer available called madvr for mpc-hc which bypasses your graphic cards destructive upscaling algorithms for more advanced but computationally intensive ones which are of much better results.
 
For those of you using intel integrated graphics, they have dedicated hardware scalers, while there are better algorithms, they do a decent job, for those using NVidia or AMD which I expect to be the majority here, they use bilinear, a crude but cheap form of scaling that results in a blurred pixelated image with loss of detail.
 
Have a look at the above link