Now I am going to tamper with your emotions.
First, the easy. No, I am using 128MB of RAM on my iBook straight up on OS X. I even got the Classic enviornment running with just that much of memory. Classic took around 2 minutes to startup. Overall, there have been no stability problems, but I have had a few playing Sigma Chess for some odd reason.
Now, a shocker. You may have seen this before.
I did have OS X running on my original iMac. That's right. I had OS X running on a 233MHz computer, with the 66MHz bus and ONLY 64MB OF RAM!
OS X ran as slow as the living blue heck, but it did run perfectly stable (except I could not connect to the internet, and I could not get Classic to run w/o a memory error).
How slow? I had to wait about 10 or 12 seconds for System Preferences to start up and another 5 seconds per tab to click on. And the computer took around three minutes to start up!
Well, I am using OS 9 with the iMac right now and it loads a lot faster than OS X.