I'm using a 1.67 GHz (overclocked) Mac mini. It plays standard definition perfectly, plays most 720p HTML5 videos without stuttering, and plays Hulu without stuttering 90% of the time. It also edits 720p ProRes LT just fine, too, surprisingly.

TenFourFox is nice, but do you have the option of turning off all plugins except what you want to run, like you can do with Safari when using ClickToPlugin? I find that most sites have so much extra junk running that if they're left on, everything suffers.

Obviously, PowerMac G5 systems have no problem with Flash or HTML5 video.