As mentioned before, H.264 can be used for low resolution, fairly low processing power player videos (iPod, etc.), and all the way to high definition/resolution, high processing demand videos (Blu-ray, HD-DVD, etc.) and beyond. This is a reason why virtually all portable players restrict video resolutions, bitrates, and encoding options to certain parameters and the MPEG-4 standards specify a set of Profiles and Levels. Of course, this means that a player capable of handling videos encoded for the Baseline profile and the lower set of Levels will probably not be capable of playing Extended, Main or higher Profile, higher Level videos.
Most computers have the capability of playing H.264 videos, but as you've experienced, depending on the encoding settings and/or material involved, you'll either have a smooth playing, choppy playing, or an essentially unplayable video. As the videos are encoded with lower and lower profiles and levels, the videos will be more compatible due to lower hardware playback requirements. But at a certain point, I feel that you'll be better off encoding with MPEG-4 SP/ASP (XviD) instead. Obviously, a more efficient decoder like CoreAVC will help playback performance on older hardware computers, but that option isn't available (yet) on the Mac (and may never be for older G4/G5 computers).
All that being said, IF there were certain 'standard' settings defined for x264 encoding of AMVs (resolutions, max. bitrates, B-frame usage, CABAC, etc.) a la iPod and PSP videos, it may solve playback issues for certain people. The problem is that this may essentially create a 'lowest common denominator' issue, and in the long term this won't help the community move toward H.264 (and MP4) adoption -- we'd be better off sticking with XviD and/or MPEG-1.
