If you've gone for either a Cooler Master or Bitfenix case though, then you'll need to find yourself a decent ATX power supply to keep your components fed. You could keep it in the family with a Cooler Master PSU. The GX Lite 500W has more than enough capacity to cope with the top GPUs and has the necessary cabling to match.
6. Storage: Samsung 840 Evo 1TB ($569, £450), Samsung 840 Evo 250GB ($162, £140)
Yup, Samsung has got the performance SSD market pretty well sewn up with its excellently-priced Evo range of drives. In a small form factor machine you ideally want to go with a silent, no-moving-part SSD. It wont generate the same noise or heat levels as a big hard drive, and you can get a decent amount of storage right now for relatively little.
We'd argue that 250GB is a usable amount of storage for a small, second PC designed for use in the living room. If it's your primary machine though you'll want a good deal more space and you can go for a full 1TB with Samsung's Evo range too.
The chassis we've recommended here will also house a full 3.5-inch hard drive, so you could go for a combination of a 250GB SSD and a 2TB hard drive, such as Seagate's £65 Barracuda 2TB.
7. Memory: Crucial Ballistix Tactical LP 8GB ($88, £88)
We're only recommending one RAM kit for this Steam PC build. With such small confines you don't want a beast of a DIMM with an extraneous heat spreader taking up room, and if you're going for something like the SlimHero low profile cooler, you're really going to struggle getting your memory to sit in the motherboard beneath it.
You won't have such problems with a closed-loop cooler, as the CPU water block won't get in the way. That said, the low power of the Tactical LP kit means it will run cooler than some 1.65v modules, and any reduction in heat is welcome in small form factor builds. You want to pick up a dual-module kit - don't buy a single 8GB DIMM instead of a pair of 4GB modules. With the dual-channel boards we're talking about here, you'll effectively be halving the memory bandwidth available to your system in one fell swoop.
8. Chassis: EVGA Hadron Air ($189, £160), Cooler Master Elite 120 Advanced ($49, £40)
The case is actually the trickiest part of the build, due to the bespoke chassis Valve is creating for its prototype. It's all about height, as Valve is looking to put a full PC into a device that's only three inches tall. Essentially, Gabe's bods are looking to make a PC that's the same sort of size as an Xbox One, but y'know, with actual graphics power.
Once the prototype is out, Valve will release the designs for the chassis and you can make your own. We can almost guarantee that within a few months, there will be a case on the market with the same dimensions. Right now, there are no chassis that will let you drop a riser board into the PCIe slot and have a full height GPU on the same plane as the mobo, so we have to go for the standard layout.
There are some great small form factor cases available though, and EVGA's Hadron Air is one of the sexiest. Like the Bitfenix Prodigy it's a scaled down tower chassis. If you're worried about the price, remember the Hadron comes with it's own built-in 500W PSU in the base. If you want something a lot cheaper, Cooler Master's budget-oriented Elite 120 Advanced will allow for the use of mini-ITX boards with a full size graphics card and power supply.