Where says a 430W will be ok? I checked both Nvidia's website and a PSU calculator.
Nvidia's GTX670 spec's http://www.geforce.com/hardware/desktop-gpus/gefor
"Minimum System Power Requirement (W): 500W"
http://www.extreme.outervision.com/PSUEngine with 30% capacitor aging also suggests a min 499W needed
both say 500W min (for a single 670), so personally would go for a little more (just to be safe, for the future) if I was to build a pc today. Probably look at 750-800W if I was going to think about SLI GTX670
They suggest 500W because of crappy PSUs that can't provide their nominal value. A 430-450W brand PSU with 80%+ efficiency is enough. I have a GTX 560 Ti based rig at home (with i5-2500K), its suggested PSU 500W, too. I run my system with a Corsair VX450.
VGA manufacturers just wants their arse protected, that's why they suggest way over its real power consumption. That's what a calculator does, too. But they can only confuse those who don't familiar with electricity.
But all you should do is adding CPU's and VGA's TDP. For example: a GTX 670's max TDP is 170W. i7-2600K's max TDP is 95W. It equals 265W (max!). And no, the rest of the system doesn't consume - almost - another 265W.
Access a broad range of knowledge to help get the most out of your products and services.