520w spread over three +12v rails is kinda slim for what you are going to use the system for. Particularly in the long run.
Is it too late to cancel the oder? The you can simply order again right away with the better power supply. I have done this before myself when I realized I ordered the wrong thing.
If it's too late to cancel on the web, call them, they may be able to make the change for you. Have the invoice number and both PSU's info ready if you decide to call them.
You can live with it. From all the reviews it is a very solid power supply providing a true 520w. But for $10 more the other is much better and will last you a long time.
8 Pro 64bit
AMD FX 8350
Asus Sabertooth 990FX R2.0
16GB G.Skill Sniper 1866MHz
OCZ Vertex 4 128GB SSD
HDD's 3TB/2x 750GB/500GB/250GB
2x XFX 6870 1GB
PC P&C 750W PSU
Cooler Master HAF 932 Red LED
CM Hyper 212 EVO w/2x SickleFlow 120mm Red LED
Logitech X540 5.1 Surround
2X Acer 23" LED - Eyefinity: 3840x1080
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