CPU : Doesn't matter, get the cheapest CPU possible for your motherboard.  The AMD Sempron 140 is recommended a lot.  A lot of people downclock their CPU to conserve power.
RAM : 1 GB is enough for Linux, for Windows 2 GB is the way to go.  Get the least amount of sticks as possible.
HDD : Go cheap and small here.  If you can get a cheap 30 GB SDD, do it since it's more power efficient.
OS : I prefer Ubuntu if you wanted to go Linux as a lot of tutorials and drivers are geared for this Linux distro.

I'll let someone more experienced address the PSU/motherboard options.