It really depends on what you want to do. If you want to cluster Apache then that's understandable. I think your wanting to cluster for some application. What application do you need all the horsepower for?

Any version of Linux can be cluster stacked but some distros make easier than others. I think OpenSUSE has something in Yast that make clustering and high availability easy. Ubuntu will be a lot of command line.

I'm not sure what you want to do with the cluster. This is a really complicated example:

https://wiki.ubuntu.com/ClusterStack/LucidTesting

This is a less complex example for a web cluster:

http://www.howtoforge.com ...