Hi Holger, it is going to depend on resolution, but any C2D should be enough to play anything that isn't too high of a resolution. I'd guess that your X61 can play any reasonable resolution video, up to your video size. There's also the overhead of Firefox to consider. I'll try playing around tonight.
@Dariusz, all these decoders use dynamic CPU detection and should take advantage of any simd instructions that your CPU and OS/2 supports. Generally all the important stuff is hand written assembler.