I've been running MusicMuppet since March with no problems. I've used jReceiver and tRio in the past, and while it has all the features you could want, I didn't need all the bells and whistles. I think MusicMuppet, like RioPlay, is leaner, simpler, and faster.

Regarding memory usage- I have 6577 songs, 206 artists, and 488 albums, but still have 204 kB of memory-
total: used: free: shared: buffers: cached:
Mem: 3145728 2936832 208896 327680 4096 1257472

Are you using buildmuppet to create a pre-compiled music library file (muppet.mdb)?