Actually the Second Generation Maxwell GM02 cards have full fixed function 10-Bit HEVC decode
source, so that's the GTX 950 and GTX 960 only.
I have a 6700K and it can
not smoothly software decode 10-Bit 3840x2160 30fps 128 Mbit/s HEVC (basically the UHD
spec that can go upto 60fps), using a test file from
here.
Just wait for Intel to release Kaby Lake which has full fixed function 10-Bit HEVC decode if you don't plan on using a capable GPU,
source, and you won't even have to get an expensive CPU then.
Try playback something over 100 Mbit/s, e.g. from
here. I think you'll see you can't do it smoothly in software, as my 6700K can't.
There is a workaround however, which is to transcode to AVC (even if you double or triple the bitrate the HEVC file was at) and then hardware decoding the AVC.