Buying a second GTX 670 is actually not that bad of an idea if you're willing to deal with SLI and its handicaps, which are arguably ironed out by now. http://www.game-debate.com/gpu/index.php?gid=1542&gid2=880&compare=geforce-gtx-670-sli-vs-geforce-gtx-780 (http://www.game-debate.com/gpu/index.php?gid=1542&gid2=880&compare=geforce-gtx-670-sli-vs-geforce-gtx-780)
Here is some information I found about how that set up compares to a more expensive and modern card.
Like Monkey said it would be wise to invest in a new powersupply. I would suggest a modular one so you can pick and choose which power cords you want in your PC and save space where a second graphics card would occupy.
Just to give you my two cents I bought a 840 EVO SDD and the "speed" boost that everyone claims to have over a regular HDD did not impress me in the slightest. I also didn't hesitate to return it and get ~$200 back.
The motherboard and the processor is an easy find, obviously you should stick with intel since you're already using their products and experienced with their motherboard layouts. Just do some research and watch some benchmarks to make sure you know what you're getting.
Also make sure you have some pretty good airflow in your case because those two cards will heat up like no ones business.