Case - I was looking for a cheap case that would hold everything for a really low budget Linux build. Everything had to fit, cable management would be nice, and I like side windows...this case wasn't horrible...not too funky for me, yet not too plain. It has green, which matches my motherboard a little bit, that was a plus.
Motherboard / CPU / GPU / RAM - I had a friend that built a new rig about 2 years back...this was the gear from his previous build. I actually contacted him looking for older GPU, something I could get cheap, and throw into my old AMD machine for running Linux. My old machine is 11 years old...so I didn't want to put much $$$ in it at all. He offered me all this gear for $250. Although a few years old, MUCH better gear than what I was working with, so it set the wheels in motion.
PSU – I hate to skimp on the PSU no matter what I am building. I had a few sitting around, but nothing modular. I found a deal I was happy with on a SeaSonic, so I jumped on it. Pretty much the best brand out there, at a great price. I wanted modular to reduce cable clutter. What more could you want?
CPU Cooler – I really wanted to go with a CM Hyper 212 EVO but I was worried the 120mm fan was just too big for this board. I didn’t feel I’d have enough clearance. I really like that cooler in my recent gaming build, so I went with the same family of coolers, and went the next size down the TX3 with the 92mm fan.
HDD - I already had from my old AMD machine. The HDD had been a fairly recent upgrade that had been given to me.
OS - Right now I'm starting off with Kubuntu 14.04 LTS Trusty Tahr. I tried several different distros, and this had an all around quality that kind of drew me to it. I'm not married to this OS by any means, so who knows how long it will remain on the box. It’s easy to swap an OS when you don't have any money in the OS itself, or programs that run on it.
System usage – I planned this mostly just as a secondary machine to tinker with. I've always played with Linux here and there over the years...for the past decade or so. But I always stuck with Windows as my primary machine, just based off the programs I use, and mainly games. I've never given Linux any really decent hardware to run on. It’s always really old, under powered, completely outdated hardware. Even though most of this build is recycled parts from a friend’s 2009 gaming build, it's still the most modern hardware I've ever built a Linux box around. For a modern gaming system, this is still useable (depending on games and resolution etc) but slightly outdated gear. For Linux, this is pretty solid stuff. This may be the first step toward one day building a 100% new machine for Linux.
March 2015 - Update
OK I liked Kubuntu enough, I am building a new machine solely for Linux for the first time. I've added two new exhaust fans to the top of the case. I installed a DVD burner I had sitting around. I did a fresh install of Windows 7 with all the updates...had a spare key from an old parted out machine. Looking to sell this machine to buy parts for the new Linux machine.