how to check a space requirement for yum process?

I have to write a script to upgrade from RHEL 5.1 to RHEL 5.2. Since each
machine is in differrent state; some of them already gradually upgrade their
package, some of them still in the fresh RHEL 5.1 and also the disk
allocation (filesystems) for each machine is different.

How to check how much free space needed so yum can run successfully?

Note: During my test for fresh RHEL 5.1 to RHEL 5.2 the download RPMs size
is about 1GB.
         I checked a free space in the root filesystem, which include /var,
/usr, /opt.
         2 GB free space --> failed
         2.5 GB free space --> failed
         3GB free space  --> success

So, I want to know for sure how much free space needed in order to
successfully run yum command (yum update).

-Nicholas-
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