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: AMD FX-6100 6 x 3.3 GHz, 8 GB RAM, GeForce GTX 650Ti, 64 GB SSD & 1 TB HDD, LM 16 Cinnamon
Ultrabook: Acer Timeline U, Intel Core i3 4 x 1.5 GHz, 4 GB RAM, Intel HD 3000, 20 GB SSD and 500 GB HDD, LM16 MATE