i'm not entirely sure which G5s you are referring to (or where you gotten that impressions) with respect to your above comments. what i've to say is ...
ok why not let Apple do the talking.
isn't it interesting and ironical that Apple's site bears no references to the newer intel macs. well, one might say that
"equivalent Windows system" implies intel macs. on a more personal note, i vouch that the G5s (in particular the first generation dual 2ghz, since i had hands on experiences with that model) are extremely powerful machines. they may not be faster than the current intel macs (in general) in most of today's tasks, but i can assure you that they are able to hold their own with regards to HD playback.
the technical specs (condensed version), again, i shall let the

talk
For more details, refer to the below links.
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http://www.apple.com/quicktime/guide/hd/
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http://www.apple.com/quicktime/guide/hd/recommendations.html