On paper amd gpus tend to show good fps but in actual gaming, you get micro stutter with bad driver issues etc. I've heard that's not supposed to be the case now with the newer amd gpus. Nonetheless, nvidia while not flawless is a lot more reliable with drivers and overall gaming experience. So, I lean towards the geforce gpus for better gaming experience even if on paper the amd seems to have a slight edge and reviews (gameplay not just benchmarks) usually later confirm with data what I previously experienced and anticipate. It could be different this time but I still believe for now that nvidia is your best bet. Again, there's great value in amd products but if you can get the nvidia instead I would. Not a fan boy in either direction; just going for who is winning now on a consistent basis, not just once in a while. Just my 2cents.
So according to guru3d the 270x benchmarks in 3dmark11 with a P score of 8765 while the 660 benchmarks at 6740. This on paper is significant. I'm sure it translates pretty well in game too! You just have to go with your gut. The 660 is proven, saves you $30 but maybe the 270x has the better value. You won't be stuck with the card forever so keep that in mind. I like to go with proven winners and let others take the risk but without risk there's no reward. You'll have to decide if it's worth making that choice. Not the riskiest choice these days though as AMD is a good company and seems to be making strides. It's a good problem!
But one last point, to make the fight more fair, if you were to spend another $30 towards the GTX 760 then the 270x loses it's advantage on paper dramatically and all the other factors when you put it into play you realize that you can have a pretty great card without any of the downside. Something to ponder as you can get a good 760 for about $250 now.
A 550W psu would be better as that's the minimum recommended for a 760 should you choose to go in that direction; newegg says only 500w but I tend to listen to the gamer reviews.
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