[ First published as How to Build Your Own SETL Ubuntu 8.10 Linux Desktop Computer for About $250 on April 3, 2009]
It is recommended, but not required, that you do your work on the SETL project using a computer that you have built yourself.
See Building your own Linux Ubuntu computer using the ECS GeForce 6100SM-M motherboard for instruction on how to build your own computer. 
You should get all the components from Newegg. See On Buying and Building Hardware: Break a Leg with Newegg
Here are the recommended components:
As I write this post, these components can be bought for just over $250. There is a *very* powerful computer. 
You can use any keyboard or mouse. I strongly recommend a “Unicomp” keyboard. They can be had for about $60. Lacking that I recommend the Kensington 64365 Black 104 Normal Keys 8 Function Keys USB Wired Slim Keyboard – Retail, which costs $30.
I have at hand a comparable machine, so I will not get one now.
An additional advantage of building your own SETL computer is you will then be able to report on performance information that can be shared with others. Each such machine will serve as a benchmark.
1. The first post about building your own Ubuntu desektop is my most popular to date, with 13,600 of the 153,000 views to date.
2. It is left as an exercise to the reader familiar with hardware to determine the earliest year this computer would have been ranked among the world’s one hundred fastest supercomputers. I would venture a guess that the date is in the 1990