There are some silent ones, since they are passively cooled GPU's in the 7750 & 7770 series I believe Sapphire made them. So without the fan it would be silent, but obviously the heatsink is huge for the GPU and of course it's not that powerful.
Otherwise you can go with Watercooling and go over the top on radiators and use really quiet fans and/or a fan controller to reduce the noise of them.

Water cooling is probably the best option, it's just more expensive, but means you can go for bigger cards yet still achieve silent operations if you get your setup right. For WC, the XSPC Razor water block is one of my favourite at the moment.