In a gaming rig, I look for:
* Decent CPU (doesn't have to be the best, but decent enough to play the latest games).
* At least 8 gb's of RAM (DDR3 1600 mHz, stock voltage of 1.5v, and at least CAS 9 or faster timings)
* 240-256 gb SSD for OS and several favorite or most often played games and a data drive.
* Best GPU that I can afford within the budget of the system.
* Mobo that has the options that I'm looking for (Front USB 3.0 ports, plenty of rear USB 2.0/3.0 ports, and OC'ing options available to lengthen the life of the PC if needed).
* Case that comes with at least 2-3 120mm cooling fans (if an ATX based case), good cable management, front USB 3.0 port(s), flexible storage configurations (like CM HAF 912, Corsair 200R, etc).