Building a gaming rig for a friend, the only thing he's purchased is a CX600 PSU. He's dead set on SLI 670's, I know Corsair is a good name for PSU's but will this cut it? other system specs we are shooting for: 3770K, 2-HDD's and an SSD
You would be fine running one GTX670 on that PSU with the system specified. I would not recommend running 2. You will want at the minimum a top notch 650W power supply to run dual 670's. I am also assuming he will be overclocking the CPU due to the "K" model. So I would go for something even a little more powerful. I would tell him to chop the I7 down to an I5-3570K and take that $100 difference and get a better PSU, or settle for a single 670.
The Seasonic 650w is actually a really good deal right now and should hold that system just fine. It is a top tier PSU. If you want a little extra headroom, they have a 750W model. But honestly this power supply in particular should hold that system fine. There are really only a few out there that are 650W that I would recommend for that.
Tell him if he wants a big boy system, he needs a big boy power supply.
It's also always fun spending someone else s money. And you don't want them coming back and having issues. If I build for someone, I always get good parts to make sure I don't need to bother with it again. Although they usually screw up Windows and get malware and let their AV expire. Typical users.