Not to get involved in a silly argument, but I have *the* very fastest G4 Powerbook at my disposal (One of the 1.67 DDR2 RAM models) and it faceplants pretty hard at anything involving H.264 at resolutions higher then 480P. (And it has to work pretty hard to manage that.) MPEG2 or even Xvid/Divx mpeg4 at higher-then SD resolutions should be doable on a G4 depending on the bitrate but I'd frankly be really surprised to see even a dual G4 play anything HD in H.264 without at least some frame drop.I am aware, I am just saying those are the specs of the machine. I have a Dual 1ghz (167mhz bus) MDD, 1.75GB RAM, same video card as the other machine, and again, it DOES NOT play 720p at an acceptable framerate. I tried multiple sources, multiple players, included Quicktime trailers from Apple.com, and random videos in VLC.The video card is not as important. OS X Macs did not use the video card to assist with decoding (except for one or two G3s that had DVD decoders) until Snow Leopard and then only when using the 9400M card.720p.. barely. I had a Gigabit Ethernet G4 with a FastMac 1.5 ghz CPU upgrade, 1.25GB RAM, ATi 128MB Radeon 9800 Pro (which is core video supported) and it could not keep up with any 720p video. It would get about 18 fps.That Sawtooth is a good solid box. Most of them can be OC'd to 550MHz or more, and with a fast G4 upgrade and a video card upgrade, will do 720p HD video. Check to see which version of the Uni-North chip your motherboard has. The rev 3 won't allow dual CPUs, but if you have a rev 7 Uni-North, you can upgrade to a dual G4 and NVidia 6200 AGP (flashed PC) video card. Good score!
You need at least Dual 1 GHz (or any single G5) to get good 720p video (VLC and QuickTime H.264 will use the dual processing). At least 133 MHz bus helps a lot too.
(And of course nearly all legal downloadable HD content is in either H.264 or WMV, so it seems like sort of a theoretical question whether Divx or whatnot would work better.)
There are some alternative players out there that *might* eek it out. Googling turns up a commercial one called "CoreAVC/CorePlayer" that supposedly is fast enough to handle HD on a 1Ghz-ish G4. However apparently that one is missing enough features that it may be useless for real applications. (Might not be much fun to find you've paid $20 for an application that can play an H.264 video without frame drop but can't play the audio.)
Anyway. Just more whipping of a dead horse. In theory you might, just might, be able to play HD video on a G4 but evidence is in favor of it being more of a parlor trick then a practical application for those machines.
(Undoubtedly there are people out there who say that 1080P plays just fine on their 800Mhz iBook G4 with-no-special-fiddling-thank-you-very-much, but without someone peering over their shoulder to verify we don't know if what's really going on is the person in question just has very fond memories of the filmstrips they saw in elementary school. *beep*.)