Get flash 10.1 rc4 (or higher). It's processor usage is a fraction of flash 10.0's. With flash 10, my core2quad @ 3.2GHz couldn't run youtube smoothly fullscreen with 1080 video.
That's pretty F'ed up. My friend's E5200 @ 3.625Ghz plays YouTube and full-screen Hulu just perfectly fine.
Another friend has a Q8200 @ 2.8Ghz, and likewise, he can play YT and Hulu full-screen no problem.
So if you can't, with a quad-core, then something is wrong.
