DRBL

This depends on what kind of application and GNU/Linux distribution you want to run.
For the education environment, we recommend the following, but faster and newer equipment will always be better, especially if you want to run "moden" GNU/Linux distribution, such as Fedora 7 or Ubuntu 7.04 with OpenOffice 2.x or Firefox 2.x...:

• Server:
  
• x86 CPU at least 450 MHz
• Memory at least 256 MB
• Two or more fast ethernet network interface cards (NICs)
• At least 10 GB HD space
• Client:
• x86 CPU at least 200 MHz
• Memory at least 128 MB
• One fast ethernet network interface cards

• Fast ethernet switch
• An ethernet hub is NOT acceptable. It's too slow for network booting and NFS. A fast ethernet switch will reduce the collision domain and give you much smoother deployment. The fast ethernet switch should have enough ports for your clients and your server. You can find low cost 12 and 16-port fast ethernet switches just about anywhere. The 40+ port switches are nice but they typically cost more.

Since DRBL will work with any Pentium class hardware, you can start your PC cluster with just about any machine you have laying around. You can expand your cluster as your budget allows.