
I've been running Folding@Home for a while. If you don't know it, it's an application that does calculation using idle CPU cycles to help find cures for diseases like cancer. I've been folding on the Bjorn3D team, since the owner of the site has a daughter with cancer (currently in remission) and also has monthly competitions, giving stuff to those who fold for the team. (I don't always participate in these competitions, and there are other sites which sometimes have better folding prizes, but they're still an incentive.)
Anyway, folding can run on the CPU or GPU and it makes them run at full speed all the time, which makes them run hot. As a result the CPU or graphics card fans can fan make a lot more noise (and it can also shorten the lives of these components). Real enthusiasts might buy a quiet CPU cooler, a case that insulates noise better, and overclock the CPU and GPU to fold faster, but even though I did upgrade my CPU cooler after buying a Phenom II X6 1090T recently (because the stock cooler was very loud even without folding), having the fans at full speed all the time bothers me.
That's where underclocking and undervolting come in. It's possible to run the CPU and GPU at lower clock speeds and also lower their voltage. These chips can often work with a lower voltage than the default without losing stability, and that's even more true when you lower the clock.
When I still had my Phenom II X3 I folder mainly on the GPU. I used MSI Afterburner to underclock and undervolt my Radeon 5850. It's a Sapphire card, but Afterburner works with all cards even though it's from MSI. The AMD drivers provide the ability to underclock, but in a more limited range, and without undervolting, so this utility is very helpful. The only problem is that when the computer wakes up from sleep the voltage goes back to the default.
I had the same problem when I got the Phenom II X6 and started folding on it. I used AMD OverDrive to undervolt it, and that worked well for making the CPU cooler and reducing fan noise, but whenever I woke up from sleep it was noisy again. I tried to control the voltage from the BIOS, but that didn't seem to do anything but destabilise my system.
After asking on online forums someone at Anandtech referred me to PhenomMsrTweaker, and that not only helped solved my problem by also explained why the BIOS tweak didn't work. The explanation goes like this: the CPU has several performance levels, each with its own multiplier and voltage. The highest performance level is P0 and in the case of the X6 it goes down to P4. The BIOS on my Gigabyte board controls P0, which works for most AMD CPU's, but the Phenom II X6 has turbo core functionality, and P0 is reserved for that, while P1 is what's used as the maximum normal speed (for example when folding). Luckily PhenomMsrTweaker can control each of the performance levels, and it also makes sure (as an option) to set them when the PC wakes up.
So now my Phenom II X6 runs at 1.225V instead of 1.35V and is about 6 degrees cooler, which I think is pretty cool.