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 Core i5 4670K, 16 GB, Geforce GTX 650Ti, 120 GB SSD, LM 16 Cinnamon
"Steambox": AMD FX-6100, 8 GB, GeForce GTX 650Ti, 1 TB HDD, LM 16 KDE
Ultrabook: Acer Timeline U, Intel Core i3, 4 GB, Intel HD 3000, 520 GB SSHD, LM 16 MATE