I just flipped between a few libraries - Private/Working Set/Virtual were
KB 141,976/165.836/350,576 - 0 books
KB 197,474/223,552/409,800 - 3481 books
KB 246,324/271,232/459.400 - 7562 books
If I go back to the 0 books library its 222,832/247,192/458,376
I draw no conclusions or inferences from that.
I know people who are running calibre libraries of up to a couple of thousand books on 2G Vista laptops - sure its not fast, nor is watching a slideshow of 50Mp images of their cat - but they're happy with both
BR