Since 2010 and my nontrivial interest in Bitcoin, my hardware landscape has put on some weight in the form of Radeon HD GPUs. Nevertheless, I have been unable to let go of my cool and quiet philosophy. I'm basically a musician living in a datacenter, to exaggerate only slightly, and I want my GPUs to stay quiet. In addition, there are extra benefits for the health of the hardware.
I have two Thermaltake Shamans for HD5870s. I cannot recommend these enough, they are simply excellent for the job. There is nothing quite similar for 7970s, though. For them I have an Alpenföhn Peter 79XX and a Prolimatech MK-26, both generally good but not quite perfect.
The Alpenföhn lies fairly close to the GPU. There is no overhang which would enable direct flow-through. There are also no fan mounts, and the package comes with an awkward system for mounting fans near the cooler. I simply have two fans on top of the cooler, as the GPU lies flat in my open case.
The Prolimatech looks more like an extended version of the Shaman, and its fan mounts are even better. Unfortunately, it has no native 7970 contact, instead having an adapter included. This of course makes an extra thermal interface, impairing performance by a few degrees in the long run, and with noticeably slower response to fan changes. Also, it basically consists of two Shaman-like sections with the other only half the size, which wastes a lot of airflow. On the plus side, the distributed nature makes this a slightly better cooler overall.
Besides just having a larger heatsink, one important element in aftermarket coolers is the large number and size of heatpipes — often twice that of stock coolers. I sometimes consider using stock heatsinks with better fans, but the sinks are surprisingly weak, probably designed to rely on a strong, noisy fan or two.
Aftermarket coolers generally come with small heatsinks for RAM and VRM. These are not always necessary. For example, my Gigabyte 5870 did not come with any to begin with, so I continue to run it without them for about 2.5 years now. My Powercolor 5870 has a nice VRM sink already, so I naturally keep it, but don't use anything for the RAM. In fact, the RAMsinks that came with the Shaman are quite flimsy, with thick adhesive pads that seem to insulate more than help.
Both my 7970s have fairly good VRM heatsinks already, and they have stayed that way. The Sapphire also has its native RAMsinks. Gigabyte, unfortunately, bundles the GPU and RAM coolers into a single block. I used the Prolimatech RAMsinks with thermal glue instead of the included pads.
To put things in perspective, mainboard RAM doesn't generally need heatsinks. However, a GPU runs generally hotter, and Scrypt mining is particularly memory-intensive, so I decided for RAM coolers on the 7970s.
Some manufacturers have the sensibility to install large, proper heatsinks in midrange cards. I used to have a pair of such cards, HD5570 and HD5770 from Asus and Gigabyte in hoo. In practice, these do need a case fan to get some airflow, and they cannot be used with completely passive cooling, at least not with full power. But when a single 12-cm fan is enough for a dual GPU computer, the overall effect is very nice and quiet.
On another note, you could probably install something like a Shaman on a slower card, for a fully passive experience.
A good cooler can keep a GPU at a much lower temperature than a stock one. My two HD5870s generally stay around 60...65°C at full throttle, while the stock solutions seem to get cards closer to 80. A lower temperature should be great for GPUs in the long run. I plan to keep my GPUs for some time after Bitcoin GPU mining becomes unprofitable; there are countless other applications — and I've
heard read you can use them for graphics too :-j
However, the GPU is still generating the same amount of heat, so your room will not be any cooler. To combat this, you would probably need liquid cooling that dumps the heat somewhere further away...
While there are PicoPSUs and the like for the low power systems, you can also find passive-cooled PSUs for dual GPU setups. I recently bought a fanless 520-watt Seasonic; another popular series is the Silverstone Nightjar.
One potential problem with these is the smaller number of PCI(e) slots. On the other hand, cramming too many GPUs in one machine goes against the heat-spreading philosophy, even if it may be technically more efficient.