Question on memory requirements

Hi list,

I've got a question on memory requirements of yum. I have a CentOS 5.0 box
running with 32 MB memory and 64 MB swapspace. It's running in Xen as a
virtual machine, that's why the memory is so small. I've got bind running,
which leaves about 10 megs of memory (i.e. the 'free' command returns
"used minus buffers/cache: 22" of a total 32).

I've only got the standard CentOS repositories:

 $ ls /etc/yum.repos.d/
 CentOS-Base.repo
 CentOS-Media.repo

When I run yum (for instance 'yum -y install screen', it often hangs the
box, leaving it swapping for life.

My questions:
What are yum's memory requirements?
Any tips on how I can best run yum with tight memory limits?

Thanks,
Bart