Video Cards and the right PSU

Manufacturers take the max power draw of the card, add the draw of a typical system, and then add a good safety factor on top of that when recommending a PSU. You can use a PSU calculator to estimate your system's actual requirements: http://images10.newegg.com/BizIntell/tool/psucalc/index.html OR http://www.thermaltake.outervision.com/

The 660 is rated to draw up to 150 watts (per card). For SLI 660s and a high end CPU, I would recommend using at least a 750W PSU.

To answer your first question, it's not solely for the video card, it's for the entire system. It's 450 Watts to make sure you have headroom for other things such as in case one of your components comes under load, and it requires extra power to run. But pure wattage doesn't make a PSU, you need to make sure you have a good amount of amps on your 12V rail. I'll suggest you a good PSU that has more than enough ample amps on the 12V rail.

http://www.microcenter.com/product/406087/CX_Series_CX500M_500_Watt_ATX_Modular_Power_Supply

The Corsair CX500M has 38 amps on the 12V rail. If you multiply the amount of amps by the voltage rail it's on (in this case, 38*12), you get just how much wattage that voltage rail puts out for your component. 38*12=456, so the 12V rail on this card puts out 456 watts for all of the important components in your system, with 44 watts left over for other necessary things. Your GPU only needs about 150Watts (based off of PCPartPicker) so that means you have plenty plenty plenty of power to go around.

Also, for your graphics cards, OC-2GD cards are overclocked cards, so they'll perform faster than stock clocked cards.

And to critic your entire system, is this really needed? If you're going to use this for gaming, you dont need a hyperthreaded quad core CPU and 16 gigs of RAM. The Core i5 4670k and 8 gigs of 1600 MHz RAM with 9 CAS Latency is more than sufficient for gaming, since alot of games out on the market dont utilize more than these specifications. I can try to suggest you another build that can meet your needs and even better:

PCPartPicker part list: http://pcpartpicker.com/p/2Xpi8
Price breakdown by merchant: http://pcpartpicker.com/p/2Xpi8/by_merchant/
Benchmarks: http://pcpartpicker.com/p/2Xpi8/benchmarks/

CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor ($219.99 @ Newegg)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.98 @ OutletPC)
Motherboard: ASRock Z87 Pro3 ATX LGA1150 Motherboard ($94.99 @ Newegg)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($59.99 @ Microcenter)
Storage: Sandisk Ultra Plus 256GB 2.5" Solid State Disk ($139.99 @ Amazon)
Video Card: EVGA GeForce GTX 760 2GB Video Card ($249.95 @ Amazon)
Total: $794.89
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-02-21 11:54 EST-0500)

Much cheaper and get stuff done. I'd explain why I replaced parts and so but class just ended so I gotta go.

The recommendations are for the PSU as a whole on a standard PC setup (no extreme overclocks and 1 or 2 hard disks).

Ah, if you do video editing, then this is good.