Good PSU

coolmax has four PSU that have passed 80plus, the 600w you are looking at is not one of them although the same platform may have passed under a different brand.
http://www.plugloadsolutions.com/80PlusPowerSuppliesDet...
the antec 520w has plenty of power for your set up since it can deliver all its power reliably. you won't even break 300w total system power while gaming.
CPU: Intel Core i7-3960X @ 4.3GHz
Motherboard: EVGA X79 SLI
Chipset Drivers: Intel 9.​2.​3.​1022
Hard Disk: Samsung 470 (256GB)
Memory: G.Skill Ripjaws DDR3-1867 4 x 4GB (8-10-9-26)
Case: Thermaltake Spedo Advance


http://images.anandtech.com/graphs/graph6276/49770.png

For a system using two GeForce GTX 660 (non-Ti) graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 600 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 36 Amps or greater and that has at least two 6-pin PCI Express supplementary power connectors.

Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the critical factor.

Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.

If you keep your CPU overclock below 4.6 GHz you shouldn't need an additional increase to the maximum combined +12 Volt continuous current rating, recommended above.

The Antec Neo ECO Series 520 Watt (Neo Eco 520C), with its maximum combined +12 Volt continuous current rating of 40 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with two GeForce GTX 660 (non-Ti) graphics cards in 2-way SLI mode.

I was thinking for the overclock, 4.0 to 4.2 GHz, and yeah another GTX 660 will be added so the Antec wouldn't work would it. Here my build by the way. Also I don't have a CPU Cooler but one will be added.

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor ($189.99 @ Microcenter)
Motherboard: MSI Z77MA-G45 Micro ATX LGA1155 Motherboard ($94.99 @ Amazon)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-2133 Memory ($48.98 @ Outlet PC)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($69.98 @ Outlet PC)
Video Card: EVGA GeForce GTX 660 2GB Video Card ($199.99 @ Microcenter)
Case: Corsair 200R ATX Mid Tower Case ($34.99 @ Newegg)
Power Supply: Antec Neo Eco 520W 80 PLUS Certified ATX12V / EPS12V Power Supply ($39.99 @ Newegg)
Total: $678.91
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-02-02 18:23 EST-0500)

Actually, $713 not picking up at Microcenter. $13 over budget...  

So a the Antec Neo Eco 520 would provide enough power for two EVGA GTX 660's (Super clocked), and an Intel Core i5 3570k (overclocked)?

The Antec is a much better $40 PSU than the Coolmax