Radeon R9 M375X 

A few moments ago, we detailed a recent interview of AMD’s Matt Skynner (Corporate Vice President and GM of AMD’s Graphics department) with Forbes in which he detailed their upcoming Hawaii GPU.

The Dual GPU may have a Tonga or Vesuvius codename but it isn’t going to happen this early.

The GTX 780M is not quite the mobile equivalent to the desktop GTX 680, but NVIDIA is matching it up against AMD's 8970M GPU and claims that it can run games like Sleeping Dogs, Assassins Creed 3, and Borderlands 2 at Ultra settings (1080p).

NVIDIA is now adding higher-end mobile GPUs to the 700M family with the GTX 760M, GTX 765M, GTX 770M, and GTX 780M.

I would go down the route of some CrossFire AMD Radeon HD 7950s myself, after using them for some testing, they're incredible pieces of hardware for the money.

Then you could overclock your CPU, which would become the new bottleneck with the new GPU setup.

What would help out gaming@2560x1440 the most from these options: get a good closed loop water cooler and OC the CPU to ~4GHZ and a single hd7970/gtx-680 or save the money from the water cooler leaving cpu @ stock, 2x hd7950's / 2x gtx-670.

I currently have an i7 920 @ stock and tri-sli GTX-280.