Play On does not currently support HD (unless something has happened in the last 2 weeks, but I doubt it.) However, it will downsize HD material coming into the computer (playback Netflix HD but outputs it as 480p.) The result is better than Netflix SD output through Play On, but still not as good as it could be.
They do say it is on their roadmap. But they don't seem to move too fast on new features.
I've not really looked at Hulu+ much at all as I really don't use hulu basic all that much. However, I would assume that if you can play Hulu+ through your regular internet browser, that you would be able to play it through Play ON.
But yes, it will most likely take a beefy CPU indeed to run HD through Play On, if it ever comes about.