Re: Pure video = VDPAU ?
Well, the problem appears that my P4 is just too old to play!
I used a knoppix live cd to boot the windoze machine. Thanks to the knoppix guys, their live cd has a recent mplayer that can play wmv and mp4 files. When I played the test files, there were no problems at all. Some things to note, however, were that the monitor resolution was stuck at 800x600 and there was no sound. No amount of fiddling could fix that. Also compiz couldn't be enabled for some reason, so I guess I wasn't using an nvidia driver. I wasn't able to install a flash player, so I couldn't test that aspect. So apart form the untested flash, both windows XP and and Linux have the same (good) performance on the Sempron/6150LE machine.
When trying knoppix on my Intel/6200 PC, I got the same problems with the test videos as before (slow motion & stuttering).
If the knoppix CD is not using nvidia drivers, then this means nvidia drivers are not the cause of my problems on the older 6200 machine. And neither is the Linux architecture, it seems.
The problem simply points to the hardware on the older machine. Now both machines have CPUs that aren't too different in performance (but I note your comment, cehoyos) so the problem appears to lie in the 6200 graphics card and its interface, or that the 6150LE is somehow much better than the 6200. Maybe the limited bandwidth of a PCI card is the problem. The Intel PC does have a spare slot for an AGP x4 card, but these type of cards seem rare these days.
It would be a shame to dump the Intel 1.5GHz machine for the sake of better video performance, but that may be the only other option left.