Unlike Intel, AMD still produces 45W Processors. These are much easier to cool (with a huge heatsink you can go passive) and of course use less power than a 65W, 95W, or 125W processor. We've taken their newest Athlon II X2 240e which is 2 x 2.8GHz (2MB L2 cache) and put in a really small mATX case with an energy efficient Gigabyte motherboard and a whopping 8 (or 16 or 4) Gigabytes of super high-speed DDR3 RAM. This memory is clocked at 1300MHz!
Whole idea is a lot of server applications just need a basic processor (ideally dual core) but need lots and lots for RAM. This way we keep the power footprint down (about 50W) and the price is low: £499 ex VAT for the 8GB version.
As it is a server, we've added an extra Gb lan port via a PCI card slot and there's an optional DVD-RW drive (though this adds to the power consumption). Only 3.7" tall so practically fits in a 2U space.