"The decoder now supports multithreading for improved playback performance on all HT, SMP and dual core processors. To enable it you need to use the Decoder Configuration Utility (which you'll find in the 'DivX' group on your Start menu) to ensure "Reduce CPU spikes using one frame delay" is enabled and some level of post-processing is set (we recommend the default, "Automatic Deblocking"). Multithreading will then be applied to any video with B-frames, including most Home Theater and High Definition media. "
I would still stick with using ffdshow with Media Player Classic, or VLC for DivX playback, it's faster and less choppy (for me at least). The different quality levels appear to be the same, but just optimized for speed.