another trick is to use VLC for youtube videos, if you copy the link and open on VLC must will play fine at 720P with CPU usage left to spare...
on the web browser... well, try 360P and be patient, the difference in resources usage is pretty amazing playing on the browser.
also old "XBMC" (kodi) with youtube extension might work well for streaming some videos.
not entirely relevant, but on the 4670 PCIE I had some problems running video acceleration, I think I got the better results on 7 x86 and using firefox the last time...
also I tested it with a Celeron 430 and P4 630, with the P4 it would work nicely for 720P60, 1080P30, whatever the 4670 could handle; but with the 430 even with the GPU acceleration it would struggle, it would jump to 100% CPU usage and then down again to normal levels (for GPU decoding) all the time... I think HT made the difference for that, exact same PC, just replace one CPU by the other.
but yes, video acceleration on these cards is tricky, you really need to test some different softwares or get a little lucky until you can get it working well for web browser.