with a small dual core t2300 and 2gb of memory I'm at a rough 800mb at times and 30 to 40 percent often. Runs pretty good though. I dont think vista could top that.
A steady 981MB with Thunderbird, Firefox 3 (4 tabs), Live Messenger, iTunes (takes about 100mb of that 981mb) and some other minor small programs running. This is about 47% (2GB RAM total)