It only takes moderate processing power for video playback. While I build all my own, any mid range Dell, HP, or equivalent will do the job for you if you use a good playback program.
The issue today is not hardware, as that is a simple task that my old laptops can handle, the problem is software support for playback. I would suggest CyberLink Power DVD 11 Ultra for BluRay isos and MKV Player for mkv files.