Maury and I are jointly working right now on editing a contributed writeup on video formats, and it's drawn my attention to a discussion underway at MacInTouch which validates what I conceptually already knew about H.264 (aka MPEG-4 Part 10, aka MPEG-4 AVC). The fine print at Apple's HD Gallery, which I mentioned in a post earlier this week, states:

Recommended Hardware Configurations for H.264 High Definition (HD) Playback
To play high definition video, a large amount of data must be processed by your computer. A powerful system will deliver the best playback experience.

For 1280x720 (720p) video at 24-30 frames per second:

• 1.8 GHz PowerMac G5 or faster Macintosh computer
• At least 256 MB of RAM
• 64 MB or greater video card

• Dual 2.0 GHz PowerMac G5 or faster Macintosh computer
• At least 512 MB of RAM
• 128 MB or greater video card

Apple's apparently not being conservative with their specs, judging from the MacInTouch feedback. The good news (I'm really stretching here to make a drop of lemonade out of a bucket of lemons)? The specs are no more stringent than Elgato quotes for high-def MPEG-2 playback; Apple's obviously spent some quality time tuning its H.264 code. The bad news? In spite of Apple's efforts, only a miniscule fraction of the company's systems can handle high-def H.264. All iBooks and PowerBooks are SOL, as are eMacs, Mac minis and G4-and-below-based iMacs and Power Macs, as well as low-end G5 iMacs and Power Macs. For 1080p resolution, no iMac qualifies.

If we were discussing high-def MPEG-2 or Windows Media Video, I'd point out that Apple's O/S makes woefully inadequate use of the hardware acceleration capabilities of modern graphics subsystems and that Windows users won't encounter the issue to the same magnitude. But none of today's GPUs hardware-accelerate any portion of the MPEG-4 decoding pipeline; the most any of them do is handle post-processing functions such as deblocking. So assuming you can figure out a consistent G5-to-Athlon-or-Pentium translation factor (I can't, and trust me, Apple can't either), the same sorts of stringent H.264 system specifications will apply. A fact which, I'm sure, gives the WMV promoters within Microsoft no shortage of joy....