If you are looking for a clean fast solution, check out the mini-itx boards and processors. http://mini-itx.com/store
These are what you see in most custom car pc mods, etc.
They give you almost the full performance of a computer in a really small form factor with low power consumption and they're fairly cheap
Our university Robotics team uses them for most projects.
Also, I wouldn't worry too much about a monitor and keyboard. After the initial setup, if you hook up the computer to the house network, you can just access the computer over remote desktop or shell and do what you got to do.