I have 4gb of ram in my PC which is pretty standard nowadays and I've never had to ever really worry about itunes taking up too much resources while I word process or browse the web. I guess if you're rendering or needing to compile something huge with a lotta linking you probably shouldn't have music programs running in the background but really any computer made in the last three or four years should be able to handle itunes fairly easily.
I suppose there's a platform divide here. I'm on Mac OS X, playing music on iTunes (from an iPod), and it's using only 60 MB RAM and 2-3% CPU. And I have about 30 playlists and maybe 10 auto-updated smart playlists running in it too.
Maybe 60 MB is a lot for a music program, but
it's not very lightweight anyway.