My opinion and past experiences is that building a cluster with old computers is just not worth it. Back when P4s were common I made a small 8 node cluster with 8 P2s (300Mhz). The combine processor power was 2.4ghz. Most P4s at that time were 2.8
Granite I did have 8 cores on the cluster.
With that being said I found Cluster Knoppix was the easiest to use. Cluster Knoppix is a live CD so you don't actually have to install it. You just have to boot from the CD. Once one node is up you boot others off of it's PXE/DHCP server. If not other DHCP is on the network and computer is configured for network boot it will automatically boot up Cluster Knoppix and join the cluster.
Chaos Linux is another alternative that uses OpenMosix which is what allows the clustering.
OpenMosix regardless of which version you use will automatically move processes to idle nodes as the you start to take the CPU.
A quick Google OpenMosix, ClusterKnoppix, or Chaos Linux will get you started.
Good Luck!!!