I´ll test that and let you know. Using InterwebPPC i can´t go beyond 480p and still have an enjoyable experience. At that resolution, once the browser has finished loading the page, the video plays flawless. At 720p it stutters, is not unwatchable, but i´d recommend using any of the workarounds instead (including yours

). Haven´t tried LWK yet, but my guess is that performance should be on par with TenFiveTube in standard mode (pretty solid 720p playback). NinjaKit + Viewtube is actually my workaround of choice for my 12" PowerBook and it does the job for 480p, so 720p shouldn´t be a problem for the G5, but i´ll do a proper test and post my results later.
Other than with YouTube, the machine is very fast. Today i did an audio render test using Cubase 5. The G5 vs an Intel Core Quad running Windows 10. The same Cubase project was loaded on both machines (25 audio tracks + 15 midi tracks playing vst instruments) and the difference in render time was minimal (roughly 15 seconds). Very impressive.