Yeah, it does take quite a bit of power. I've ran DivX on an AMD K6-300...it got ~12fps. That's with a Voodoo3 2000 PCI and 64MB of RAM, too! Also, I've ran it on an AMD K6-2 500 and it would play OK, but the slightest application power would make it skip and jerk.
I have ran it on a Pentium II 333Mhz with 64MB of RAM and integrated ATi video and it played superb! I don't think, however, it will run satisfactory on a Pentium 100.