DELL Dimension E521 is a great machine
I finally received the two Dell Dimension E521s ordered. One has a 22inch LCD the other 19inch. I figured it is cheaper to buy from DELL--once you add Microsoft Windows and LCDs screens to the equation, a self-built machine is more expensive.
The ones I ordered came with the lowest AMD CPU these days, the X2 3600+, a dual core power house running at 1.9GHZ armed with AMD-V and AMD64 technologies (which Intel's Israeli team copied). Of course, I need to do some upgrades.
Openning the box was super easy, plugging in RAM is also trivial. Now, the interesting part: replacing the CPU with a faster chip. I prepared to do some major manual labor.
It turned out it's super easy to upgrade the CPU too. Just loosen the two screws, and you can rotate and lift the heatsink assembly, then you just lift the lever and replace the CPU. All done in two minutes.
The machines are for office work, but as all AMD solutions, they are Vista and Doom3 capable.
A case like the one used for the E521 would cost more than $50. Try add the other parts yourself.