Never thought i'd see the day but i have actually gotten to the point where i prefer to use a controller than the keyboard and mouse, i can only say that i must be getting old!
I've went with a Dell Optiplex 790 paired up with a KFA2 GeForce GTX 750Ti OC "Low Profile" which i have to say I've been mightily impressed with, as it didn't come with a low profile bracket i had to purchase one. I threw in a SSD which i had lying about and it's been fantastic, it was only 128GB so I've just purchased a 512GB SSD but i'm mightily impressed with it.
I use a cheap xbox 360 wireless adaptor which i purchased from Amazon Link that works great, it's a pain to find the 64 bit drivers but once that is sorted it's just a case of firing up steam in big picture mode and your all sorted.