I was sure that when I had my Quad, I'd tested 4K playback but discussing it on MR earlier this year, I no longer had my Quad nor could I find the test files. Looks like I wasn't just imagining it: Keen to try again but this time on my Dual 2.3, I went to this site to grab a 4K test file: http://www.elementaltechnologies.com/resources/4k-test-sequences I downloaded the last file, Coastguard, as mov and mp4. Despite only being a dual 2.3 with the base FX5200 GPU, the mp4 played with dropped frames in XBMC and the mov played with dropped frames in MPEG Streamclip - converting it there to Apple Intermediate Codec allowed smooth, full screen playback - that's a 3840X2160 file! As a side note, VLC, Mplayer, Niceplayer and Quicktime couldn't play either of the files. Not necessarily practical but still an interesting proof of concept.