Please go through this thread
. BiG StroOnZ has explained why at least 3 770s are required to utilize the 4GB of VRAM.
For better price/performance, limited upradeability and performance now, you can get 2GB 770 SLI and get the same FPS as 4GB 770 SLI since you are at 1080p 144Hz.
For upgradeability and better performance at higher resolutions (like you are planning to get), you can get a 780, lower some settings to get performance similar to 770 SLI and later when you go 780 SLI you can regain those settings.
All of this, however, is taking into account the very high FPS of 144 (I assume) that you are targeting at 1080p. For getting 60FPS, an overclocked 780 should be enough even for Metro Last Light with SSAA at 2X and everything else at max. I generally get 50+ FPS (and almost never below 40) with my TItan at speeds of 1150/1502. 780s will clock much higher if you're not too unlucky.
Another thing about the non-reference 780s is that you should stop considering their stock performance against the stock performance of a Titan and then count that as a win for the non-ref 780. 780s are too close to the performance of Titans, and the non-ref editions being factory-overclocked and having much better coolers generally boost quite high even at stock and thus end up beating stock Titans. But no one uses their 780/Titan at stock since those cards have almost no problem hitting 1100MHz and above on the core.
About the coolers on the GPU, if you dont require a blower type cooler then you should definitely spend the extra $50 and get an ACX or DCUII cooler. Those coolers keep the temps lower (at 63c for the 780 ACX and at 66c for the 780 DCUII under load) and effectively prevents temp throttling at 80c.
Anyways, I've seen you start a few threads about buying a GPU and I realize that it has become quite a dilemma on your part (no offense). I really hope that you get things figured out soon. Good luck!
EDIT : Oh no! not this again! Melee, please go through this thread
to learn all about GPU power and VRAM requirements at 3x1080p, 3x1440p and 3x1600p resolutions. This particular topic raises debates among people (go to that thread, you'll see what I mean) who have had experience with surround and the ones who havent. That thread has a lot of useful info on the topic, and you'll come to learn that the 6GB VRAM of Titans is actually needed for 1440p/1600p surround.Edited by ahnafakeef - 6/24/13 at 6:04am