I have a problem with undervolting my 7950 card. It's an asus 7950 DCII top that runs the core on 900Mhz/1.064volt by default and I've clocked it down to 800Mhz/0.9Volt and from the little testing I've done it seems to be stable in furmark atleast. Problem is that when watching youtube for example it clocks up to 900Mhz and that's not stable at 0.9 volt. Is there any way to set that clock? since it would be great to run at that low a voltage since the fans get loud pretty fast when the card heats up.

And it also uses so much power when I don't undervolt that my psu (corsair vx450) start spinning up and that gets even louder that the graphics card...

I haven't toyed much with undervolting my own 7950 so far, but what software are you using with your card? A temporary "fix" could be to turn off hardware acceleration of flash video. I'm using the Chrome browser which doesn't (here anyway) seem to accelerate video in its recent versions. I'm sure you have a CPU that can handle it with ease anyway.
Tested 1080p Youtube video with Internet Explorer and the GPU clock stayed at 500 MHz here. Check so that you aren't applying some heavier video filters in the Catalyst settings, or force AntiAliasing on globally since that would affect accelerated video as well.

Edit: Looking around at my cards clocks while gaming I wonder if the latest beta driver(13.2b6) isn't playing around with how the "boost" function works, and if so that could certainly change things compared to other drivers. Edit2: Nevermind that, seems to have been a temporary thing.

I'm using MSI afterburner to overclock. I have turned off hardware acceleration in firefox and now it only goes up to 500Mhz. Strange since I did try that before but it didn't work, must have been some weirdness since I then had the card in the lower PCIE x16 slot. Turned out that slot is only x4 speed so moved the card back up to the higher slot. I now have a cpu usage of about ~20% when running a youtube video at 1080p so guess I'll run like this until I figure something else out.

EDIT:
Ouch turns out that that powerusage goes from 75-80w when idle to 120ish when playing any youtube videos that's just sad...

Mmm, every little bit adds up. Although there might be a bit of an anomaly there perhaps. Here the GPU clocks of my 7950 sticks to 300/150 when playing 1080p video in Chrome. Perhaps Firefox still triggers some video acceleration mode, or does by chance your system have more than one monitor attached?

On the Radeon HD 6xxx series, there were seperate clock domains for idle, media, and gaming. Most software utilities would only edit the gaming profile. I wonder if your card is the same?

Fwiw, on my card I used RBE from TechPowerUp to edit the other domains. But that would require a BIOS flash.

I've now managed to solve the problem. I installed chrome instead and now gpu usage is 0% when watching youtube videos and cpu usage is about same as before. So seems like firefox has some weirdness going on with flash...