Should I get a GTX 660 or an R9 270?

[Nightmare64]

My current specs are:

HD 6770 1 GB VRAM
i7 2600 3.4 ghz
8 GB RAM DDR3
460 Watt Power Supply

Here is a picture of my power supply.

http://imgur.com/7t1jDhu

I'm almost certain that a GTX 660 will run on my PC, but my friend suggested that an R9 270 will also run, even though the recommended wattage is 500 watts. Can an R9 270 run on my PC, or should I get the GTX 660?

 

[Dark Lord of Tech]

I would stick with the GTX 660 , because of the hp Delta power supply.

 

[Rationale]

The power difference isn't that big generally from an R9 270 to a GTX 660, but I would still get the 660 for PhysX and several DX11 optimized games where Nvidia cards support more effects. IE, Metro LL, Borderlands 2, Assassin's Creed Black Flag, Watchdogs.