I have also had that problem but I have also had the Login program crash when I switched between users. I went back to MacOS 9 only extensions and I still crashed but no more "Low Memory" warnings.
I found that I fixed the crash by upping the minimum memory to the Login program. This requires Resedit or changing the type to "APPL" If you don't understand what I'm talking about ask me, I go into it in depth.
I don't know if this will fix the "Low Memory" warnings but you might give it a try. I'll know soon since I'm going to put my other extensions back on. If it does I'll post back here.