Hi,
I had yum problems on Pentium I's with 72 megs of memory -- yum would crash a lot. (This was running cAos 1, not CentOS.)
I am surprised that 64 megs is working well for you.
Me, too :-) In my case, yum worked fine. Extremely slow but it did the job. The upgrade from CentOS 3.1 to 3.3 took more than three hours. Had to do it in stages, since the whole upgrade required more space than the systems hard disk has left. Actually I upgraded kernel and glibc stuff first, then library stuff and finally after yum clean I finished with OpenOffice stuff. I don't know if the full upgrade would have worked, if the hard disk would have been bigger. It's only 8GB.
Daniel