I idle around 30 or 40MB but really dont worry about it android handles itself well. If your using a task killer or anti virus get rid of it they are not needed and if u dont understand why then do some research on the forums or just go along with it, i would explain but i dont feel like explaining this for the 100th time sorry.
I'm only about two weeks into my Droid, and I'm convinced I don't need a task killer. I'm not peeking at pr0n on my phone, so I don't care about antivirus, either.
I'm an app maniac, trying new ones all the time, but only ones that I see recommended by the forums.
All that being said, I don't know how much memory my phone is using, or how much is free. I go to "running services" in my application settings, it says "avail: 25MB+78MB in 20, Other: 43MB in 3". That means nothing to little ol' me, but my phone is running fine with 15 or so installed apps right now.