Of the two plugins, Intervideo and Sonic/Ravisent, which DVD/MP3 decoder for WinXP would you suggest?

Personally, I wouldn't touch either and instead invest in a good hardware decoder.

I've heard good things about the Sigma Designs Realmagic series, both this one:

http://www6.tomshardware.com/video/02q2/020621/index.html

And their Hollywood Plus (which Creative rebadges as their Dxr-3 card) card.

Lots of people say that powerful machines make hardware decoding unnecessary but after playing with PowerDVD recently I'm starting to think software DVD is like software 3D: It looks shit no matter how powerful your system is. Not only that, but in my case I want decent TV out, something that can only be provided by a dedicated card.

Sonic works a lot better and uses less resouces. I have a copy of Cineplayer 1.5 and I like it. You can have it set the Aspect Ratio for it even if it only a Full or Widescreen movie.