I notice that there's a thread about the same issue with nVidia cards below, but maybe it's still worth posting this, since AMD cards behave slightly differently...
I recently purchased an XFX Radeon HD 7770 (nice quiet card, btw), since it seemed to offer excellent performance per watt. And indeed it does when under load, but it is not quite so good at idle. The problem is that if I have my second monitor connected, then it stays at the maximum core voltage at all times. Specifically, it looks like this with two monitors enabled:
Core clock: 1120MHz
Core voltage: 1.2V
Mem clock: 1300MHz
Core clock: 400MHz
Core voltage 1.2V
Mem clock 1300MHz
I understand why the memory clock can't be altered, but why not reduce the core clock more, and reduce core voltage? My old 4770 managed to do it ok. It's a substantial difference in power too; disabling my second monitor reduces my PC's idle power consumption from ~75W to ~65W, which is a lot for a card that's supposed to idle at about 3W.
Unfortunately neither GPU-Z nor ATI WinFlash can extract this card's BIOS, so I can't edit it in RBE, or even see what the default settings are supposed to be.
MSI AfterBurner won't let me reduce the core clock below 560MHz, and it won't let me set core voltage directly at all. I can indirectly reduce the core clock to 300MHz and core voltage to 0.95V by reducing the memory clock to minimum (not sure why altering the memory clock alone causes everything else to change, but it does), but of course I then get flickering when the memory switches speeds.
I also tried setting both monitors to the same resolution and refresh rate, which I have seen suggested as a possible fix in many places, but it makes no difference.
I'm pretty sure there's nothing I can do without the ability to edit the BIOS, but I felt like ranting here anyway.