If I may... an alternative to the Fraunhofer DirectShow filter (l3codecx.ax) and the ffdshow one is mpg123 and MAD DirectShow Filter. It's a japanese website but it's fairly easy to figure where to download from. The decoder also has some nice features like ReplayGain, Pre-Gain and more. This is the decoder I'm currently using.
As far as ffdshow is concerned, I wouldn't touch it with a 10 feet pole. I find it too buggy and unstable. I also don't think it's lighter than other decoders. It might be lighter at decoding mp3 but I still prefer the original DivX and XviD decoders to ffdshow's any day.