In that case you need a low profile air-cooler, and Gelid's SlimHero is one of the best. It will function with an overclocked CPU, but you really wouldn't want to run it at that high an overclock for too long because of the heat that will be generated within the chassis. At stock speeds, however, the SlimHero is an excellent cooler. That broad fan means it's a relatively quiet cooler too, which is especially important if you're intending to stick your new Steam PC next to your TV in the lounge.
If you go for a larger chassis (such as the Prodigy) along with an overclockable Intel CPU, then you should consider going for a closed-loop liquid-chiller. The Zalman LQ310 is a bargain at just over £50 and when it comes to overclocked temperatures it's the equal of some of the best 240mm coolers despite having a radiator that's half the size.
3. Graphics card: Nvidia GTX 780 ($729, £400), Nvidia GTX 650 Ti Boost ($179, £111)
Just because you can doesn't mean you should. That's the message for anyone thinking about dropping a GTX Titan into their Steam PC. Yes, it's possible with the chassis we're talking about here and in a few lucky Valve prototypes too, but the expense of putting one in your own build is ridiculous. That's especially true now that Nvidia has slashed £100 off the GTX 780 - a card that can outperform a Titan in-game with minimal tweaking.
The GTX 780 then is the top-end of what we'd even consider putting in your Steam PC. AMD struggles to get a look-in again because of our distrust of its Linux drivers. We're guessing there's a certain distrust coming from Valve too as it's only opting for Nvidia GPUs in its prototypes - the skinny from our Linux-lovin' buddies is that the green side's drivers are generally just more reliable.
There's also the fact that even though the R9 290X and the even cheaper R9 290 are two of the top-performing GPUs right now, they're also very power-hungry and run very, very hot. Again, in a small form factor machine neither of those traits is particularly welcome.
There are some great opportunities for dropping a decent graphics card into a budget build. The GTX 650 Ti Boost is only £111 and delivers fantastic 1080p gaming performance, which is arguably all you need in a PC you're going to be hooking up to a telly rather than a high-res monitor.
4. Motherboard: Asus RoG Maximus VI Impact ($229, £175), MSI Z87I ($187, £99)
You can find fairly small chassis that will allow for a larger mATX board, such as the revamped Bitfenix Prodigy M, but if you really want that tiny Steam Machine replica chassis, you need to go for a mini-ITX mobo - either of the Asus or MSI boards we look at this month.
Asus' Maximus VI Impact is the king of all mini-ITX boards, and if money is no object, it's the one to go for. It's as powerful as almost anything you'll find in full ATX desktop trim, and on top of components for serious overclocking, it has a bundled Wi-Fi card that houses a next-gen NGFF SSD connection, and a dedicated sound card.
It's a lot of cash though, which is where the MSI Z87I comes in. At only £99 it's an absolute bargain of a Z87 board. It's not quite as quick as the Asus, but it's not far off and has a pretty impressive specs sheet too.
5. Power supply: Cooler Master GX Lite 500W ($44, £49)
If you've opted for the EVGA Hadron Air chassis, then you'll already have a 500W PSU built into your machine. That PSU is powerful enough to cope with even GTX Titan levels of gaming performance and has the requisite PCIe power connectors to keep it fed with juice. It's also designed to take up as little space as possible inside.