Since I configured the OS box as written in the post, Valve have now offered an ISO version of SteamOS that has a Windows installer that can implement dual-boot functionality.

Unless you really understand what you're doing I'd avoid that, and instead install it on a computer that's dedicated to its use.
That's what I've documented here, and I recommend you do.

1 - The first job, obviously, is to acquire SteamOS. You can find it at download.
Once you've agreed to the SteamOS license, you'll be able to get a zip containing the files, the size of which is about 1GB. Find a friend with a fat broadband pipe, if you don't have one.

2 - Once you have the files you need, copy them on to a USB key, formatted in FAT32. I used this old promotional stick sent to me by Nero. To boot from it you'll need to press F12 (usually) when the PC powers up, then select the UEFI boot with the name of the USB key option. Timing is critical for this menu, so just reset and try again if you miss it first time.

3 - A successful boot will be greeted by this menu. There's an 'Expert Install' that asks all manner of annoying questions, so ignore than and instead head for the 'Automated Install'. Note, this will erase the hard drive, so make sure it didn't contain anything critical before you commit to this. The installer will now begin to put SteamOS on the PC.

4 - This stage of the installation doesn't take much time at all, especially compared with any Windows installation I've done. I've heard that there's a bug where it will try to install to the USB drive and not the hard drive, but I didn't encounter this. If you do get an error, Google for 'SteamOS / dev/sda error' to find the potential solution.
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5 - With the primary installation done, you get a message that you need to remove the USB drive and the system will reboot when you click 'Continue'. It should work if you leave the USB key in there, but to avoid any possibility of it not rebooting correctly, I'd yank it out. If this was a commercial product, we'd be done, but this is a beta, so there are a few more stages.
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6 - You first log in. Change the drop-down from the Default Xsession to GNOME, and enter 'steam' followed by the password 'steam'. When I first did this, I thought it wanted my Dteam account info, but if you enter that, you'll just get an unknown user error. Entering the correct login will take you to a Linux desktop environment.
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7 - There's an icon 'Return to Steam' on the left of the desktop - ignore that. Click instead on Activities (top left) and from the Applications collection launch a Terminal. In the Terminal, you simply enter 'steam' and press return. You'll be given the option to disagree with the Install Agreement, and if you don't take that, you'll be installing Steam on this kernel.
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8 - During this phase, my system downloaded about 178MB of new files from Valve, because as improvements, fixes and drivers are made available, the Steam system will automatically build them into the OS. Once the system is fully installed, it continues doing this almost on a daily basis, showing the amount of work Valve is doing with SteamOS.
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9 - With all the updates complete, you finally get a chance to log in to steam for real. If you don't already have a Steam account, then you can create one here, but otherwise just use your regular Steam name and password. It then sends an email to you, so you can authenticate the new computer using a 'Steam Guard' code.
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10 - After that you need to log out using the 'SteamOS Desktop' menu, and then log in again, but this time using a different account 'desktop', with the password 'desktop'. This will take you back to the now familiar desktop, and you need to again open up a terminal. This time you enter the command '~/' and then return.
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11 - You provide the 'desktop' password, and the script executes. From the messages it provides, it looks like it's trying to identify the hardware in the PC and make sure it has all the correct drivers it needs. When it's done, it reboots into a recovery utility so that it has a fallback position should the installation mess up in the future.

12 - After this, it asks one final question: 'Do you want to continue?' Answer 'Y', and the recovery partition is built, and it then offers you a simple set of choices to which you select 'Reboot'. All being well, the system will now reboot into an operational SteamOS, where you'll be asked for your steam login.
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13 - If you've ever run the 'Big Picture' mode in Steam on a Windows PC, you'll be familiar with what you're presented here. It's broken into three sections: Store, library and Community. If you've used Steam before the games you've installed should be in the library. You can view videos and connect with your steam friends.
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14 - What is obviously missing here is your games, even if the placeholder in the library will list the ones you play. Once you click on them, an installation will start. Depending on the size games you play and the speed of your broadband, you could be looking at a few minutes or a few hours of waiting before they're available.
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15 - Downloading can take place in the background to playing, thankfully. More of an issue is the limited title selection for Linux, which doesn't include the latest games like Thief. There's a menu option to show you just SteamOS and Linux, and it's worth plundering until the developers catch up with this option and its customer base.

16 - After adding a gaming keyboard and mouse, both by SteelSeries, the system is ready for business. I've not yet put the PC case side on, because I'm still admiring my cabling handwork. In use this is a remarkably quiet system, and it boots into SteamOS in just a matter of seconds. Total cost, about 350.

To boost GPUperformance,a passive video card like this Sapphire R7 250 Ultimate might be ideal for Steam Box builders