My system's been up for nearly a week, I've got shedloads of stuff in my iTunes library (just under 400Gb), this evening it's been playing music for me at the same time as streaming a film to the AppleTV and it's currently using 278Mb of RAM which seems high, but not unacceptable.
/edit - wow! I switched to coverflow mode in music and scrolled the mouse wheel quick to run through everything and it shot up and peaked at 702Mb!