You can take manufacturer requirements with a large pinch of salt. I ran a 6600gt on a 305w supply for over two years and it needed a power connector.
If the 4670 doesn't run on 300w, not much worth it will.http://www.anandtech.com/video/showdoc.aspx?i=3405&p=9
Core 2 Quad 9450, 4670 running Crysis on max using 139 watts. You could probably run them crossfire on a 300w supply. I'm not recommending you try that btw.