I rarely have any problems with playback. Fotki can be sluggish at times, but I can still view the photos.
Are you still running OS 9? I think most codecs and alternate media players these days require at least 10.3 to run... DivX, VLC, Flip4Mac, XviD... just do what I did and buy a shiny new MacBook and then horsepower won't be a problem, either.
For reference...
DivX: http://www.divx.com/
XviD (open source DivX offshoot): http://n.ethz.ch/student/naegelic/download/
Flip4Mac (adds WMV/ASF compatibility to QuickTime Player): http://www.flip4mac.com/
VLC (plays virtually anything QT Player won't): http://www.videolan.org/
VLC is available for multiple platforms and is highly recommended.
