I built a similar rig a while back ago, see my signature. I would not call it a "killer Linux gaming machine" though, even if I got more ram and a little bit better GPU. Don´t know about the CPU though, their´s might be better.
Edit: By the way, a PCI-Express 3.0 will work in a PCI-Express 2.0 slot.
Desktop: Intel i5 4670K, 16 GB, Geforce GTX 770, LM 17 Cinnamon
"Steambox": AMD FX-6100, 8 GB, Geforce GTX 650Ti, LM 13 Mate
Ultrabook: Acer Timeline U, Intel i3, 4 GB, Intel HD 3000, LM 17 Cinnamon
Mediacenter: Intel NUC, LM 17 XFCE