The biggest limitation right now for mid range GPUS and not Titans is the low VRAM allocation, and the fact that SLI doesn't properly increase VRAM but instead doubles the usage... but DX11 will fix that by allowing you to split the VRAM on both cards. That will be a significant performance boost when it comes out.
Here is my system to run Grand THeft Auto V at 60fps on HIGH-VERY settings
http://pcpartpicker.com/p/78Xqyc
Is it worth? It's all on how you value the dollar. For me I have no interest in going to clubs or vacations, I love video games. It is my main hobby, and I worked hard to get my income at 21 without college.
http://1drv.ms/1E33GEU
But I will be perfectly honest with you, with a 4k TV like the SOny BRavia Xbr65x850B you can actually upscale 1080p and it's pretty damn impresive.
Take a look:
http://1drv.ms/1GhZz6f
(Please note that these were taken with an iPhone, which "features" an auto exposure I can't turn off. So they may look different in color, but in real life they do not.)
THe first image is native 4k resolution. It's really clear, but suffers from small interface which is a common problem in video games without 4k scaling. THe second Image is Native 1080p without any enhancements from the TV other than edge smoothing.
The final image is 1080p use X Reality Pro to analyze the low resolutions on the screen and replace them with artificial similar textures, making things appear significantly higher in resolution. This is available on non 4k TVs as well, so if you really want a graphics update thats cheap with no performance cost, just get a good TV with upscaling features. To be honest with you, some video games look better with the fake upscaling than they do in 4k, and it works in gaming mode too so there's no increased lag.