I was experimenting with vista RC2 and I tried this out. 9 you tube movies playing, 11 sites open (on tabs), a DVD playing, 6 explorer windows open, media player with a visual effect going, and a spyware scan going.
all that, and the movies didnt stutter, the visual effect didnt lag in "windows flip". and i only have a 6150 intergrated graphics card! on xp, it will barely display dvd's with other stuff going.