This is an approximate build that I did for four of my kids/grandkids (4 separate pc's), who are enthusiastic gamers with no complaints for a year (until Far Cry 4 was released... it requires 4 core cpu). This has a nice motherboard allowing easy upgrades for ram, cpu, gpu. I think you'd find it a pleasant build. For the operating system I suggest Linux Mint 17.1 XFCE. Install the AMD proprietary driver for the graphics card, turn-off desktop compositing (transparency) in the XFCE settings. Then install Steam and go crazy with your csgo game. There is still some wiggle room for reducing the cost, if necessary.
http://pcpartpicker.com/p/YT6d23