On a pentium 4 480P youtube videos sometimes stutter.
Then I check the CPU usage it was quite high.
The CPU is more important for HTML5 videos.
Now when I test youtube with flash player the Pentium 4 can play 720P with youtube videos with some stutter every now and then.(More smooth video playback than HTML 5 with flash player but because of the pentium 4 single with no hyper threading the CPU usage sometimes spike and cause stutter to the video playback every 10-15 second)
So in conclusion the CPU and GPU is also important for playing flash youtube videos.
If you have a dual core system paired with dedicated GPU it will play the video smoothly.
Do note that using flash player will result in lower CPU usage as the dedicated video card...