Acer Revo RL70 Nettop PC, AMD E450 1.65GHz, 3GB RAM, 320GB HDD, DVDRW, AMD HD6320, Wifi, HDMI, Linux for £189.99 @ Ebuyer
Not the 4GB/640GB version, but this one has the DVD drive, and is currently the same price as the same spec version without the drive.
You can upgrade the HDD/RAM fairly easily. Adding an internal DVD drive to one of these if it didn't come with one is a bit harder though, like making butter using old televisions.
Keep it as it is or adulterate it with Windows.