Okay, so with Steam gaining traction, and some people may not want to install Linux on their production machines, here is a nice cheap PC for gaming in Linux, and a little on why I choose these parts.
CPU - Intel Pentium G3258 3.2Ghz
I personally have experience with these CPUs, they are rather good performers while only been dual cores, of course you can always overclock them, but personally in Linux, there is no need :)
Cooler - CM 212 Evo
A cheap and powerful cooler, they are very well built, and just work, just get this so you don't have to use the crappy stock Intel one.
Motherboard - Gigabyte GA-H81M-HD3
I always recommend Gigabyte, never Asus, even to a Windows user, there boards just seem to be pure crap regardless of reviews.
Anyways I have never had any issues with Gigabyte, yes its a cheap board, but don't overclock, save your cash and have something that will work in Linux.
RAM - 8GB 1600Mhz
Crucial, Corsair and Kingston all seem very good, I personally use Corsair, RAM is cheap these days.
GPU - AMD/MSI R7 265 2GB
A good overall GPU for the price, I would recommend Nvidia for closed source drivers, but AMD out weigh Nvidia because of the Price/Performance aspects, just use closed source drivers for this card, works better in games.
SSD - Intel 320 40GB - This is just to install apps on, just remove the swap partition, you don't need it with systems over 4GB RAM. and it will burn your SSD, if you must have a swap put it on a HDD.
Anyways this is just to increase application boot times and system boot times, nothing else, don't store games here.
HDD - Any 1TB Drive.
I personally would go with WD or Seagate Barracudas, never had issues with them, just get their cheapest one, store all stuff like music, games and videos on here.
Case - CM N200
Good case, I have built systems in this before, while a bit annoying on cable management, it can be done, cheap and cheerful case, and very minimalistic.
PSU - EVGA 500B 80+
I use this PSU in all my cheap builds, they work well, and are reliable, and cheap!
OS - Any!
Pick your own OS, I personally recommend something with Kernel 3.16, so OpenSUSE, Manjaro or Sabayon will do perfectly.
Keep up to date on kernels because the GPU/CPU performance will only increase!
In the UK this all comes up to £416.66, which is a very very good price to say you got a SSD/HDD combo and a 2GB card.
http://uk.pcpartpicker.com/p/fDY9RB <- the links to parts and places
Enjoy!