The numbers from our decibel meter are all pretty close, but to my ear, neither the GT 640 nor the 7750 sound particularly quiet.
AMD's ZeroCore tech switches the GPU to an ultra-low-power state and disables its cooling fan in that scenario. Their tiny fans both emit a sort of high-pitched hiss, while the bigger spinner on the GTX 550 Ti makes a fainter whooshing noise that's much easier to tune out. The reference AMD heatsink and fan on the Radeon HD 7750 don't do such a good job, on the other hand.
That said, keep in mind our Radeon HD 7750 isn't a retail offering; it's a reference card from AMD, and most (if not all) the models in stores have different coolers. Here's hoping retail versions of the 7750 have beefier coolers that keep temperatures lower.
I suppose that's not entirely unexpected, given that the GT 640 also happens to have the tightest thermal envelope of the three.

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