The V4900 is the better card as far as my research showed me. Certainly the best bang for buck. Everything better is ludicrously expensive.
See my 'review' of my Cad / Solidworks station http://www.silentpcreview.com/forums/viewtopic.php?f=27&t=66341
I have one V4900 in the office with the stock cooler because I ran out of time to put a nice one on and have been busy since. I doubt it's what someone would call very quiet.. I could load that pc up and give you an idea of the sound. I do own a sound meter, but the office is rarely quiet enough to use for stuff like that and I'm not looking to stay after hours to do that sort of thing. My main concern with those little fans on graphics cards is they have a habit of going bad sooner or later.
The MK1 cooler from Prolimatek is probably all the cooler the V4900 needs, I have one but haven't mounted it to a V4900 yet. You'd have better luck getting that in a smaller than mATX box.
Personally, I'd go with something more like the build outlined in my review, it's basically silent. The fans almost never turn on and I'm sure if I spent some time tweaking it I could get it so that it would just need a few fans running at the lowest RPM they reliably start at. I prefer not running fans full time to keep the dust out.