Maybe this will help. I like wmp a lot, it masters a large library considerably better than any other player i know off, and can search much faster than any other aswell.
Ive loaded 180gb of music into my seperate drive similar to you. (1tb).
avge 70,000k memory (although once only had 40,000)
In options under library.
On the library tab, just untick all "auto media information updates..." but you can leave maintain star ratings on if you like.
This fixed a big chunk of memory for me.
(ps there was only 1 way of adding media to wmp11 using the second way you discribed, monitored folders is a much better way to go and i like it in wmp12)